Data from the second National Health and Nutrition Examination Survey (NHANES II) were analyzed to estimate the prevalence of oral contraceptive use in the United States, 1976-80. The overall unadjusted prevalence of oral contraceptive use was 16.7 per cent for premenopausal females ages 12-54 years (19.2 per cent for ages 15-44 years). Approximately 8.7 million females (95 per cent confidence interval, 6.9-10.5 million) were oral contraceptive users at the midpoint of NHANES II (March 1978). Comparison to the NHANES I, conducted in 1971-74, indicated a stable number of overall oral contraceptive users in the US population during the 1970s, with shifts in certain age groups: oral contraceptive use increased for females ages 12-19 years and decreased for females ages 20-49 years. The overall age-adjusted prevalences indicated a 2 per cent (95 per cent CI, 0.2-3.8 per cent) decline in oral contraceptive use from the early to the late 1970s. The NHANES provides comparative data and supports findings from another national survey showing a decrease in the per cent of females using oral contraceptives during 1973-82. Trends in oral contraceptive use are also presented by race, poverty level, rural-urban residence, marital status, and education level.