Measurement of the 17-hydroxyprogesterone concentration in blood spots was used to identify cases of congenital adrenal hyperplasia among patients with inappropriate virilisation, or salt wasting, or both. Between 1978 and 1986 61 were identified among 707 patients (278 neonates, 204 infants, and 225 children). The incidence of classic congenital adrenal hyperplasia was calculated for a seven year prospective trial period using the blood spot 17-hydroxyprogesterone method in selective screening. There were 38 salt losers and 14 simple virilisers in 968,303 live births, an incidence of congenital adrenal hyperplasia of 1:18,000 in the Hungarian population. Selective screening led to earlier diagnosis of congenital adrenal hyperplasia and a pronounced decrease in mortality. A central laboratory to measure the blood spot 17-hydroxyprogesterone concentrations is valuable for the investigation of patients at risk for congenital adrenal hyperplasia in countries where blood steroid assays are not readily available.