Leydig cell development in humans, although for years described as being biphasic, with fetal and adult phases of maturation, is better considered as a triphasic developmental phenomenon. The morphological literature is summarized in this commentary. Although the majority of studies are of a qualitative nature and many questions remain as to the relative and absolute numbers of cells involved in these developmental phases, this literature is more consistent with a triphasic developmental pattern. This view of Leydig cell development is in accord with the well-known triphasic history of testosterone production, i.e. peaks at 14-18 weeks of fetal life, 2-3 months after birth, and from puberty throughout adult life. It is also significant that the neonatal phase of testosterone production is dependent upon reactivation of the hypothalamic-pituitary-testicular axis (HPT). The current interest in the functional implications of the neonatal period will be better served by considering human Leydig cell development as triphasic.