Autopsy data on 1,206 children and adult patients with acute myelocytic leukemia (AML) (585), chronic granulocytic leukemia (CGL) (204), acute lymphocytic leukemia (ALL) (308), and chronic lymphocytic leukemia (CLL) (109) obtained from 1958 to 1982 were reviewed. This analysis has shown that, whereas the proportion of patients with residual AML at any anatomic site decreased significantly and uniformly over the entire study period, significant corresponding decreases in patients with CGL and ALL occurred only since 1976 and 1978, respectively. No significant corresponding decreases were noted in patients with CLL at any time. Significant decreases were also noted over time in the rates of extramedullary site involvement by AML, CGL, and ALL. Whereas the lymphoreticular organs, kidneys, adrenals, and pituitary were most often involved at autopsy by CLL, the testes, leptomeninges, dura mater, uterus, large bowel, and pancreas were most often involved by ALL. In general, patients with AML and CGL showed the lowest relative rates of involvement of the various organs by leukemia during the 24-year period. Whereas patients with AML and ALL showed significant decreases in the rates of involvement of nearly all anatomic sites during the most recent study periods, those with CGL and CLL showed corresponding decreases in only a few organ sites. The lower rates of organ involvement in patients with AML and ALL attest to the more aggressive eradication of leukemic cells by therapeutic regimens in these diseases over time. In particular, the significant decrease in the rate of meningeal involvement by ALL during the most recent period is probably attributable to central nervous system prophylaxis.