Background: In cases of a missing maxillary lateral incisor, an implant-supported crown is often the treatment of choice in order to avoid affecting intact adjacent teeth; however, proper implant placement requires a specific minimum amount of alveolar ridge bone volume. In general, alveolar ridge development is depended on tooth development and eruption; consequently, tooth loss has major impact on alveolar ridge dimensions and tooth agenesis seems to impair proper alveolar ridge development. It seems thus reasonable to consider that the impact of tooth agenesis on alveolar ridge development might depend on the gap width, i.e. the distance between the neighboring – regularly erupted – teeth. In particular, the influence from the adjacent teeth on alveolar ridge development would be diminished with an increasing mesio-distal gap distance, which in turn might result in deficient alveolar ridge dimensions, i.e. reduced height and bucco-palatal width in the center of the edentulous alveolar ridge. Aim/Hypothesis: This study aimed (a) to evaluate whether there is a correlation between the mesio-distal gap size and alveolar ridge dimension in patients missing the maxillary lateral incisor either due to agenesis or loss; (b) to determine the edentulous alveolar ridge dimension in the region of the missing maxillary lateral incisor and simulate straightforward implant placement; and (c) to assess any effect of tooth agenesis on the alveolar ridge dimension at the adjacent teeth (i.e. central incisor, canine). Material and Methods: Per protocol, 3 groups (n = 40 per group) were planned, including patients (1) with agenesis of one permanent maxillary lateral incisor (TA); (2) with the maxillary permanent lateral incisors regularly erupted but lost >3 months prior to the CT scan (TL); and (3) with the maxillary permanent lateral incisors regularly erupted and in situ (control; C). The following parameters were manually recorded by a single calibrated examiner: (1) mesio-distal gap width between the central incisor and the canine; (2) average bucco-palatal alveolar ridge width in the coronal (1st to 5th mm) and apical (6th to 10th mm) part of the alveolar ridge; (3) alveolar ridge bone area from a level 1 mm below the top of the alveolar ridge and 10 mm apically; (4) alveolar ridge height; and (5) possibility of straightforward implant placement (10 mm long x 3 or 3.5 mm in diameter). Further, clinical data regarding the actual treatment performed, i.e. bone grafting prior to- or in association with implant installation, were retrieved from the dental records of the patients of TA. Differences in alveolar ridge dimensions (i.e., bucco-palatal width, area, height) among groups [TA vs. TL vs. C) groups were assessed by One-way-ANOVA with LSD post-hoc test; correlations between the alveolar ridge dimension and the mesio-distal gap width were tested by the Spearman correlation coefficient. Results: Altogether 104 maxillary CT-scans were evaluated; the intended sample size of 40 was not achieved for TL due to frequently uncertain history of tooth loss. The area and bucco-palatal width of the alveolar ridge at the lateral incisor and at the adjacent teeth was significantly reduced in TA compared to TL and C (Table 1). Further, in TA, but not TL, an increasing mesio-distal distance between the adjacent teeth resulted in a significantly reduced bucco-palatal width of the coronal part of the edentulous alveolar ridge (r = −0.464, P = 0.003). This impeded simulated straightforward implant placement in >50% of the cases in TA, even with a reduced implant diameter of 3 mm. This simulation had good agreement with the actual treatment performed. Based on clinical data from 26 patients of the TA group, straightforward implant placement was not possible in 58% of the cases. Conclusions and Clinical Implications: The present results indicate that the wider the mesio-distal gap is in maxillary lateral incisor agenesis sites, the thinner bucco-palatally should the alveolar ridge be expected to be. In particular, a mesio-distal gap of >6 mm precluded straightforward implant placement in 60–80% of the cases. These results are relevant for treatment planning, since additional hard and/or soft tissue augmentation procedures should be frequently expected for optimal functional and aesthetic outcomes.