Previously unavailable time-depth acute leukemia incidence data were obtained from the Connecticut Tumor Registry for the years 1935–82 and for ages 0–84 years at diagnosis. Acute non lymphocytic leukemia (ANLL) incidence has risen over the period of observation. Acute lymphocytic leukemia (ALL) incidence has risen much less dramatically. After adjustment for cohort effects, the age-incidence relationships of the acute leukemias show several remarkable features. Incidence of both ALL and ANLL is higher in males than in females in childhood but not in adulthood. Incidence of ALL is higher than incidence of ANLL in childhood, while the reverse is true in adulthood. After a fall in incidence in young adulthood, both ALL and ANLL show age-incidence curves that are similar to those of most adult carcinomas: Incidence increases as a power of age. These features of the age-risk relationship may reflect the growth characteristics of the normal hematopoietic stem cells at risk, and this growth may be influenced by exposure to infectious agents.