Twenty-seven patients (14 female, 13 male; 3 pubertal) with growth hormone receptor deficiency (Laron syndrome) were treated with recombinant insulin-like growth factor I (IGF-I), 40-120 micrograms/kg body weight b.d., for up to 12 months. Height SDS was between -9.1 and -3.2 at the start of treatment (age, 3.7-22.9 years). Before treatment, most patients had increased basal serum concentrations of growth hormone (2.4-208 mU/l) and low serum concentrations of IGF-I (< 20-69 micrograms/l), IGF-II (69-295 micrograms/l) and IGF binding protein-3 (0.16-1.59 mg/l). In all but the two oldest patients, the growth rate increased by more than 2 cm/year compared with that before treatment. Asymptomatic hypoglycaemia (blood glucose < 3.0 mmol/l) was recorded in ten patients in 0.7% of measurements. Four patients experienced symptomatic hypoglycaemia. A transient asymptomatic decrease in serum potassium occurred in most patients after injections.