Longitudinal studies in 19 girls and 17 boys with congenital adrenal hyperplasia were carried out to correlate the degree of control with serum concentrations of testosterone (T), Δ4-androstenedione (Δ4) and dehydroepiandrosterone. All serum values were compared with those of normal children in the same pubertal stage. Good or poor control was judged by growth rate, bone age advancement, signs of virilization, and urinary 17-ketosteroid excretion. DHEA was suppressed in all treated patients whether in good or poor control. Serum T and Δ4 were within the normal range for boys in good control. Serum T and Δ4 were within normal limits for pubertal girls in good control but were below normal for prepubertal girls. Serum T and Δ4 in girls in poor control were elevated in all pubertal stages. In prepubertal boys in poor control, serum T was elevated. However, in pubertal and postpubertal boys in poor control, serum T was in the lower range of normal. Δ4 was the single androgen elevated in all boys in poor control. Therefore, an elevated testosterone concentration was indicative of poor control in all girls but in boys only before puberty. In pubertal boys serum T was normal and did not discriminate between good and poor control. However, elevated Δ4 concentration indicated poor control in both boys and girls at all stages of puberty. In conclusion, we have studied serum androgens in children with congenital adrenal hyperplasia as a means of evaluating treatment. The single androgen that best reflected good or poor control was Δ4.