The effect of insulin (10--10 000 mU/1) on the proliferation of cultured human arterial smooth muscle cells was studied. Smooth muscle cells were cultivated by explanation. Cells from the third to the fifth subculture were used. Proliferation was studied by growth curve experiments. Insulin stimulated cell proliferation in all concentrations (p less than 0.001). Growth was however stimulated more by a medium containing 10% fetal calf serum. The highest concentration of insulin produced only 35% of the effect of 10% fetal calf serum. Our results support the hypothesis that insulin may play a role in atherosclerosis.