Three children with psychosocial growth retardation increased their serum somatomedin C levels from 0.20 ± .02 to 0.81 ± 0.2 U/ml in response to exogenous growth hormone. The pattern of somatomedin C response to growth hormone was no different from that seen in 11 prepubertal children with growth hormone deficiency who received growth hormone according to the same acute treatment regimen. This result suggests that the poor growth response of children with psychosocial growth retardation to exogenous growth hormone is not due to an inability to synthesize somatomedin C, and that it may represent a degree of resistance to the actions of somatomedin C.