Little is known about the number of women who identify as lesbian. Estimates from the US range from 1% to nearly 10%. Accurate estimates are critical in order to meet lesbian's healthcare needs and to address health problems that may be more prevalent among them. This study used capture-recapture methods to estimate the lesbian population of Allegheny County, Pennsylvania. Mailing lists from four sources were used to identify lesbians. The capture-recapture method and log-linear modelling were used to estimate the number of lesbians in the defined geographical area, and the percentage of the female population they comprised there was determined through census data. Allegheny County, Pennsylvania, USA. A total of 2185 unique names were identified. The capture-recapture method estimated that the total lesbian population of Allegheny County was 7031 (95% CI 5850 to 8576). Therefore, based on the 1990 census figures, the county's adult lesbian population was estimated to be 1.87% (95% CI 1.56% to 2.28%) of the adult female population. An estimate of the lesbian population is fundamental for addressing lesbian's health needs and for developing appropriate research programmes. Capture-recapture methods have the potential to provide accurate and reliable estimates of this population in any location.