Background. Growth hormone (GH) in combination with conventional therapy in hypophosphatemic rickets (HR) has been shown to promote renal phosphate (P) conservation and to result in a better metabolic control. This study aimed at investigating the acute biochemical effects of GH in 7 patients (5 female, 2 male) with HR aged between 2.16 and 16 years. Methods: Each patient received the following in a sequential design: oral P plus 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] therapy to determine the optimum doses for baseline requirement followed by GH-only therapy and GH +1,25-(OH)(2)D-3 therapy and GH + P + 1,25-(OH)(2)D-3 therapy each for 2 weeks with 1 washout week off treatment in between. GH was given at a dose of 0.03 mg/kg/day s.c. on a daily basis. The dose of oral P used ranged between 500 and 2,000 mg/day, and the dose of 1,25-(OH)(2)D-3 ranged between 0.25 and 0.5 mug/day and was kept constant for each child throughout the study. Results: Laboratory investigations repeated at the end of each treatment, and the first washout period showed that the serum P level was highest (2.9 ng/ml) during the GH + P + 1,25-(OH)(2)D-3 period with higher serum 1,25-(OH)(2)D-3 levels: 50.9 +/- (SD) 23.4 ng/l. Parathyroid hormone and alkaline phosphatase levels did not show a significant difference between the periods. The tubular P reabsorbtion rate showed an insignificant increase during GH therapy periods. Conclusion: Considering the fixed dose of P and calcitriol, it may be concluded that GH added to conventional treatment in HR resulted in a slight improvement in the biochemical parameters without any side effects at the short term. Copyright (C) 2001 S. Karger AG, Basel.