Serum IGF-I levels were measured in 14 patients (9 children and 5 adults) with Laron syndrome (LS) during long-term treatment by IGF-I. Recombinant IGF-I (FK-780, Fujisawa Pharmaceutical Co. Ltd., Japan) was administered once daily subcutaneously before breakfast for 3-5 years to the children and for 9 months to the adults. The initial daily dose was 150 micrograms/kg for children and 120 micrograms/kg for adults. Before initiation of treatment the mean overnight fasting levels of serum IGF-I in the children was 3.2 +/- 0.8 nmol/l (mean +/- SEM), rising to 10 +/- 1.7 nmol/l during long-term treatment even on a dose of 120 micrograms/kg/day. The serum IGF-I levels 4 hours after injection rose from 31.2 +/- 3.5 to 48 +/- 2 nmol/l. In the adult patients, the initial basal IGF-I was 4.1 +/- 0.7 nmol/l, rising to 16.1 +/- 3.84 nmol/l after 8-9 months treatment. Serum IGF-I levels at 4 hours after injection rose in the adult patients from 24.1 +/- 5.8 up to 66.8 +/- 15.4 nmol/l. A progressively increasing half-life during long term exogenous administration of IGF-I to patients with Laron syndrome was demonstrated by following serum IGF-I dynamics after injection. Based on the fact that no antibodies to IGF-I were detected and on findings in previous studies, it is speculated that the increasing serum IGF-I levels during long-term IGF-I treatment are caused by an increase in serum IGFBP-3 induced by chronic IGF-I administration. It is concluded that treatment with IGF-I necessitates regular monitoring of serum IGF-I levels; in patients in whom the age adjusted maximal levels are exceeded, a reduction of the daily IGF-I dose is indicated to avoid undesirable effects.