The commonly occurring short stature in the condition of thalassaemia major was investigated with respect to the possible role of the somatomedin growth factor low molecular weight non-suppressible insulin-like activity (NSILAs). Nineteen affected patients (12 boys, 7 girls) aged between 2 and 21 years were studied. Twelve of them were on or below the 10th centile for height of whom 7 were on or below the 3rd centile for height. Serum immunoreactive growth hormone responses to exercise were normal in 9 of 11 subjects tested. Using an isolated fat cell bioassay NSILAs was undetectable in 10 and was more than 2 SD below the normal mean value in the other 9 subjects. High molecular weight NSILA (not a growth factor) was very low or undetectable in all 9 subjects tested. Low molecular weight NSILAs did not show the normal correlation with age in childhood, nor was there any correlation with height, height velocity, or bone age. The 2 children above the 50th centile for height had undetectable NSILAs. There was no evidence of iron or ferritin interfering in the bioassay, and mixing experiments showed no evidence of inhibitory activity towards NSILAs in thalassaemic sera. Low circulating levels of the somatomedin NSILAs may contribute to the short stature in thalassaemia major, but other factors may permit normal growth in some affected children.