The effect of recombinant human growth hormone, hGH, treatment on Ca metabolism and growth is being investigated in patients with Osteogenesis Imperfecta Type III in collaboration with J. Marini, HDB. Fractional absorption of dietary calcium and the rate of Ca distribution within the body is studied in these patients before and after treatment with hGH. Preliminary results of from 5 subjects show an apparent decrease in fractional absorption of dietary calcium (0.29 +/- 0.19 pre treatment vs 0.14 +/- 0.11 post treatment). At the same time an increase in both the whole body exchangeable pool (TEP = 265 +/- 95mg/kg pre treatment vs 673 +/- 616mg/kg post treatment) and rate of calcium accretion to bone (Vo+ = 94 +/- 29mg/kg/d pre treatment vs 142 +/- 75mg/kg/d post treatment) was observed. While suggestive, the variance in the mean values of these results is too great to be significant at the present time. Further studies to confirm these observations are underway.