County data on the percentage of people without health insurance are seldom available, although state program planning requires such information. As part of an evaluation of Washington's Basic Health Plan (BHP), we conducted a telephone survey in nine Washington counties to estimate the percentage of people under the age of 65 who were uninsured. We used regression analysis to estimate the percentage uninsured in a county as a function of the percentage unemployed. Two validation approaches yielded very good results, suggesting that the equation could be used to estimate the percentage uninsured in unsurveyed counties. The variation ranged from 15% to 23% uninsured in the 9 surveyed counties, and was estimated to range from 9% to 35% among the state's 39 counties. With proper caution, estimates based on this equation can probably be used in other states if better data are unavailable.