Rate of fingernail growth (FNG) of the middle digit of the right hand was assessed in 46 healthy pigtailed macaques at approximately 6-month intervals for 4 to 6 years. Mean FNG for 24 females, ranging in mean age at measurement from 7 to 24 years, was 104 mu/day; the mean for 22 males, ranging from 7 to 20 years, was 111 mu/day. Longitudinal analysis showed that FNG declined in animals of both sexes and that the rate of decline (-4.8 mu/day/year) did not differ between them, i.e., the mean regression coefficients (beta) of the two sexes were significantly different from zero and not significantly different from each other. Results of cross-sectional analysis differed from those of longitudinal analysis in that the mean FNG appeared to be more rapid in males than in females, particularly in the older animals. Inter- and intra-individual variability in FNG did not increase with age. FNG appears to be an excellent functional marker of the rate of aging because the direction of change is essentially decremental, a significant degree of change can be detected in a few years, and the rate of change is relatively constant across the adult lifespan. In addition, measurement of FNG is innocuous, quantitative, inexpensive, and simple.