The effect of 25-hydroxyvitamin D (25OHD), given orally during the reversal of hypomagnesaemia, was studied in five patients with hypomagnesaemic hypocalcaemia and low serum levels of 25OHD and 1,25-dihydroxyvitamin D (1,25(OH)2D). The results were compared to those obtained in five other patients with similar initial levels of magnesium, calcium, 25OHD and 1,25(OH)2D who did not receive 25OHD. Serum levels of 1,25(OH)2D in the ten hypomagnesaemic patients were lower than in ten control subjects with low serum levels of 25OHD. The reversal of hypomagnesaemia was similar in the two groups of patients and elicited a similar increase of circulating iPTH levels. The expected increase of circulating 25OHD was observed in patients supplemented with 25OHD; their circulating 1,25(OH)2D rose within 48 h to normal levels, contrasting with the delayed and poor increase of 1,25(OH)2D in patients receiving no 25OHD. The evolution of serum calcium was however identical in the two groups. Our results suggest that vitamin D deficiency was a significant factor leading to low circulating levels of 1,25(OH)2D in hypomagnesaemic hypocalcaemic patients. The biological consequences of low serum 1,25(OH)2D in these patients remain unclear, but clearly, normal levels of 1,25(OH)2D are not essential for the correction of hypomagnesaemic hypocalcaemia.