Objectives: To compare the alveolar bone dimensions in patients with lower second premolar (P2) agenesis prior to and after primary molar loss on CT scans, and assess the possibility for straightforward implant placement. Methods: Alveolar bone dimensions were evaluated on 150 mandibular CT scans in three groups: (i) agenesis of P2, with the primary tooth in situ, and regularly erupted first premolar (P1) and molar (M1) (AW); (ii) agenesis of P2, without the primary tooth in situ for ≥3m, but regularly erupted P1 and M1 (AWO); and (iii) P1, P2, and M1 regularly erupted (CTR). The possibility of straightforward placement of an implant 3.5 or 4.3 mm in Ø × 10 mm long was digitally simulated and compared to the actually performed treatment. Results: Buccolingual width (7.3 ± 2.0 mm) at the coronal aspect of the ridge in the AWO group was statistically significantly smaller comparing with both the AW (9.2 ± 1.4 mm) and the CTR (9.5 ± 1.1 mm) group; width reduction appeared to be mainly due to “collapse” of the buccal aspect of the ridge. Simulated straightforward placement of implants with a diameter of 3.5 or 4.3 mm was possible in 62% and 56% of the cases in the AWO vs. 86% and 84% in the AW group (p = .006 and .002, respectively). Straightforward implant placement was actually possible in all patients (22) in the AW group, while 28% (11 of 39) of the patients in the AWO group needed additional hard tissue augmentation. Conclusions: Significant dimensional differences exist in the alveolar ridge, especially in the coronal part, at lower P2 agenesis sites missing the primary tooth for ≥3m, when compared to P2 agenesis sites with the primary tooth in situ. It seems thus reasonable to advise that the primary second molar should be kept as long as possible, in order to facilitate straightforward implant installation and reduce the probability of additional bone augmentation procedures.