Serum somatomedin (SM) activity by bioassay was measured in 46 idiopathic pituitary dwarfs before and during long-term hGH treatment. The mean (± SD) pretreatment SM activity was 0.38 ± 0.17 U/ml. The mean (± SD) pretreatment annual growth rate was 3.4 ± 1.0 cm/year. None of the patients had a SM activity within the normal range of age-matched controls. The mean SM activity 1 day post GH was 1.11 ± 0.28 U/ml. Neither the mean SM activity nor the growth rate during the entire treatment period up to 8 years decreased with the duration of the treatment, always being more than 1.0 U/ml and 5.0 cm/year respectively. With respect to normal data for chronological and bone age separately, 43.3% and 60% of the SM activity in the patients during hGH treatment attained the levels of age-matched normal controls. The increase in SM activity in the patients with treatment was not related to their growth rate. SM activity measured by bioassay is useful for evaluating clinical response to hGH therapy.