In a consecutive series of 15 male adolescents and young adults with congenital adrenal hyperplasia (CAH), the size, shape, firmness and echostructure of the testes were assessed. The latter was abnormal in 7 patients under long standing treatment with glucocorticoids (group I). In 8, 5 under and 3 off treatment for several years, ultrasonography (US) was normal (group II). On the basis of the US findings the patients were placed in two groups. In group I, the testes had a heterogeneous ultrasonographic pattern; clinically, most felt hard and irregular, although their volume was normal. Sperm count in 3 patients was 5.0 to 14.4 x 10(6)/ml. Five patients have 21-hydroxylase deficiency that was diagnosed in early infancy and had salt-wasting; two have 11-beta-hydroxylase deficiency that was diagnosed late and had no salt-wasting. In group II, testicular volume, shape, firmness and echostructure were normal. All have 21-hydroxylase deficiency, no history of salt-wasting and were diagnosed late. Sperm counts in 3 patients off treatment were 10.5 to 66.0 x 10(6)/ml. In severe cases with a history of salt loss, TALT with deficient spermiogenesis seems likely despite treatment. In mild cases, TALT is absent and spermiogenesis may be normal even without treatment. US is much more accurate in assessing the testes than palpation.