Serum levels of pregnancy zone protein were measured in children with growth hormone deficiency and in girls with Turner syndrome, before and during treatment with recombinant human growth hormone and in healthy controls. The pregnancy zone protein serum levels in growth hormone deficiency patients before treatment were significantly higher than in controls (median value 2420 μg/l vs 434 μg/l; p≤0.001). In Turner syndrome patients they were within the normal range. The administration of rhGH to both growth hormone deficiency and Turner syndrome patients resulted in a significant decrease in the serum pregnancy zone protein levels by approximately 50%. The addition of 50 ng·kg−1·d −1 ethinylestradiol to the growth hormone treatment in Turner syndrome patients led to an increase in pregnancy zone protein concentrations in four out of five patients. Elevated pregnancy zone protein levels were also found in two children with growth hormone resistance (Laron type dwarfism). In one patient with placental growth hormone deficiency, pregnancy zone protein serum levels during pregnancy were within the normal range. These results suggest that the serum pregnancy zone protein levels are down-regulated by growth hormone.