The effects of recombinant human growth hormone (rhGH) treatment for three years were compared in patients with achondroplasia (ACH) and hypochondroplasia (HCH), whose diagnosis had been confirmed by DNA analysis of the fibroblast growth factor receptor 3 gene. Height SDS (H-SDS) and height velocity SDS (HV-SDS) using the standard for ACH significantly improved during three-year treatment as compared with that before treatment in both ACH and HCH except HV-SDS in the third year. The improvement was much greater in HCH than in ACH. The mean increase H-SDS using the standard for ACH in three years in ACH (from -0.2 SD to 0.1 SD) is almost negligible but that in HCH (from 1.2 SD to 2.6 SD) can be estimated as effective clinically. It can be concluded short-term GH treatment in HCH is effective to increase growth rate and H-SDS, but it has little effect in ACH. Further studies would be required to confirm the other beneficial effects of GH treatment such as increase in bone mineral density in ACH and HCH and the effect on the final height.