A total of 33 patients (17 female, 16 male) with Laron syndrome (n = 31) or hGH-1 gene (n = 2, type IA deletion) from 22 centres in 12 countries were enrolled in a study conducted by Pharmacia & Upjohn, Stockholm, which was designed to test the efficacy, in terms of growth promotion and safety, of IGF-I (Igef(TM)). The patients were treated with 40-120 microg/kg IGF-I s.c. twice daily after meals. After the study ended, the patients continued to be treated on an individual basis. The results of 17 patients, who were treated for 48 months or longer were available for the present analysis. Six patients were treated for up to 72 months. When treatment started, the mean age of these patients (8 female, 9 male) was 9.1 (3.7-13.5) years and mean height was -6.5 +/- 1.3 SDS. At the end of the observation period, the mean age of the 17 patients was 14.2 (9.1-17. 7) years and mean height was -4.9 +/- 1.9 SDS. All patients showed a significant increase in growth during the final year on IGF-I, with two of them reaching the age-corresponding 3rd centile. The total gain in height (DeltaHT) was 1.7 +/- 1.2 SDS. DeltaHT SDS correlated negatively with age at onset of treatment (R(2) = -0.78, p < 0.02). BMI was 0.6 +/- 1.8 SDS at start of treatment and 1.8 +/- 1.5 SDS at the end of observation. Total DeltaHT SDS correlated positively with total DeltaBMI SDS (R(2) = 0.59, p < 0.01). Long-term treatment of patients with GHIS thus proved to be effective in promoting growth. If treatment is started at an early age, there is considerable potential for achieving height normalisation. The treatment modalities need to be optimized with respect to the growth-promoting and metabolic effects of IFG-I.