Few studies of human cardiac myocyte proliferation in the perinatal period have been conducted. We measured the proliferative activity of left ventricular myocytes in tissue obtained at autopsy in three surgically induced abortuses, 20 preterm infants with gestational ages ranging from 12 to 35 weeks, eight term infants with ages ranging from 1 day to 11 months, and five adults. The preterm infants lived less than 24 h, thus simulating the in utero condition of developing hearts. To assess the proliferative activity of the myocytes, we measured immunoreactivity using the monoclonal antibody MIB-1 against the recombinant Ki-67 nuclear antigen. Immunostained sections were examined by light microscopy, and the results expressed as a staining index (SI) of 0-3, according to the percentage of positively stained myocyte nuclei. Myocyte proliferative activity remained constant during the early preterm period and decreased in the late preterm and early postterm periods. Adult myocytes, regardless of cardiac weight, did not reveal proliferative activity as assessed by immunostaining. This proliferation pattern is consistent with findings in most earlier studies in animal models.