In order to clarify the role of 1, 25-dihydroxyvitamin-D [1, 25-(OH)2-D] in calcium stone formation, correlations between l,25-(OH)2-D serum levels and levels of calcium, phosphate and uric acid (UA) in serum or urine and between the l,25-(OH)2-D serum level and age were analyzed in Ill formers of urinary stones (57 Ca stones) and in 44 controls. Furthermore, 1,25-(OH)2-D serum levels as well as levels of Ca, phosphate and UA in serum and urine from formers of different urinary stones and controls were compared. 1,25-(OH)2-D serum levels were determined by a radio-receptor assay based on a calf thymus receptor. l,25-(OH)2-D serum levels correlated positiviely with urinary UA excretion in formers of Ca stones and with urinary Ca excretion in formers of Ca stones and controls. In formers of Ca stones urinary Ca excretion was higher and serum phosphate was lower than in controls. 1,25-(OH)2-D serum levels did not differ. l,25-(OH)2-D serum levels decreased with increasing age in controls, but not in stone formers. Our results show that urinary UA and 1,25-(OH)2-D in serum co-influence Ca stone formation, and they suggest that there is an increased sensitivity to l,25-(OH)2-D in formers of Ca stones.