Studies on human postmortem material report lower brain weights in older than in younger cohorts, whereas there is no apparent change with age in the rhesus monkey. In view of these contrasting results, we examined the pattern of brain weight across the life span in the chimpanzee, one of the closest biological relatives of humans. To place the study in context of the empirical life expectancy of the chimpanzee, we first performed a survival analysis on data from 275 chimpanzees that were maintained in the colony of the Yerkes Primate Center. The survival analysis revealed the maximum life spans of female and male chimpanzees to be about 59 and 45 years, respectively. We examined fresh brain weights from 76 chimpanzees ranging in age from birth to 59.4 years of age. The brains were taken from 9 infants (birth to 1 year of age), 25 juveniles (1-7 years), 13 adolescents (7-15 years), 21 young adults (15-30 years), and 8 old adults (over 30 years). Adult brain weight was achieved by the age of 7 years. The adolescent and young adult chimpanzees had the largest brain weights; in these two age groups combined, the mean brain weight (+/- standard deviation) was 368.1 g (+/-37.3) for females (n = 17) and 405.6 g (+/-39.4) for males (n = 17). This sex difference was statistically significant (P < 0.01). Simple linear regression performed on the combined material from females and males aged 7 years and older revealed a decline in brain weight with advancing age of 1.1 g/year (P < 0.05). When the effect of sex on brain weight was statistically controlled for, the loss of brain weight with age was 0.9 g/year (P = 0.07). These results suggest that brain weight declines moderately with age in the chimpanzee as it does in humans.