Using a nonintrusive procedure for sleep monitoring, four 24-hour recordings of 40 independently living women, 65-94 years, were made during a 3-month period. Although the subjects distributed their sleep over much of the day, all measures showed significant reliability: with four recordings, reliabilities (r44 values) were > 0.70; and with only one recording, reliabilities (r11) were > 0.40 [r11 for the respiratory disturbance index (RDI) was lower but still significant]. Within-individual variability over recordings was low for total sleep time (TST), sleep (S), and sleep efficiency (SE) and high for sleep latency (SL) and RDI. The mean TST was 7.6 hours, which is not markedly different from that of younger adults. Within the age range studied, there was no relationship between age and any of the variables, including the amount of within-individual variability. These results, using the Home Monitoring System (HMS), indicate that both stability and instability of sleep parameters characterize the sleep of older women. The nature of specific forms of individual variability needs to be explored as possible indices of aging as distinct from indications of neurobiological abnormalities.