The objective of this study was to estimate the prevalence of self-reported physician-diagnosed systemic lupus erythematosus (SLE) in the USA. During the conduct of an epidemiologic study of systemic sclerosis (scleroderma), 16 607 randomly selected telephone numbers in the continental USA were called to recruit controls. All potentially eligible women were asked 'Have you ever been told by a doctor that you have lupus or SLE'? Of 4034 women aged 18 and above who completed telephone screening, 15 stated they had a physician diagnosis of SLE, corresponding to a prevalence of 372 cases per 100 000 (95% confidence intervals: 208, 614). After review of available medical records in six cases, however, the prevalence of 'validated' SLE was revised to 124 cases per 100 000 (95% confidence intervals: 40, 289). In conclusion, these data suggest that the prevalence of SLE in the USA may be up to three to 10-fold greater than previously estimated. Based on 1990 census data, we would project that over 275 000 women aged 18 and above have SLE in the USA.