Agenesis of mandibular second premolar is reported to be 2.4-4.5%. The diagnosis can be set on at average 9 years of age. Early treatment in the form of extraction of the second primary molar and in some cases also the remaining three second premolars and comprehensive orthodontic treatment are often a good treatment solution. In vertical deep bite cases, cases with spacing in the dental arch, mandibular posterior rotation and for extractions disadvantageous growth pattern, a treatment with retaining of the primary molar must be taken in consideration. When there is no or minor infraocclusion, root resorption less than half of the root length, and no caries or fillings at the age of 12-13 years, there is a good prognosis for longtime survival of the primary molar.