The number and size of juxta-adrenal cortical cell nests or nodular formation (JACN) and their histology were examined in autopsy material from 255 successive subjects of various ages (from fetuses to elderly patients). The study of number and size showed that JACN occurred significantly more frequently among subjects aged 0-year-old (0Y0) to 10Y0 (p < 0.01) than among other age groups. The study of numbers within each nodular size range (large nodular formations [LNF; nodules with a minor axis measuring 0.25 mm or more] and small nodular formations [SNF; smaller than LNF]) showed that the number of SNF and the SNF to LNF ratio decreased according to age, while the number of LNF remained relatively constant. Immunohistochemical examination of proliferating adrenal cortical cells using the mouse anti-human antibody MIB-1 demonstrated many positive cells in subjects with many JACN. No nodular proliferation of adrenocorticotrophic hormone positive cells occurred in the adenohypophysis. Fibrous thickening of the adrenal capsule increased gradually with age or medication. JACN was very frequently observed among subjects with conditions such as asplenic syndrome, acute lymphocytic leukemia and Down's syndrome. These data suggest that JACN might be an anatomic variant but that certain factors could increase the size and number of JACN after birth.