Normocalcaemic male stone formers, 31-51 years old (n = 108) on a free diet, were divided into a hypercalciuric group (n = 47) with calcium excretion rates higher than 8.0 mmol/24 h, a normocalciuric group (n = 32) with calcium excretion rates below 6.1 mmol/24 h and an intermediate group (n = 29). There were no statistically significant differences between the hypercalciuric and the normocalciuric groups with respect to serum levels of calcium, phosphate, creatinine, urate, ALAT, albumin, PTH, 1,25-dihydroxyvitamin D or urinary excretion of cAMP. The group of patients with high calcium excretion had significantly higher serum levels of 25-hydroxyvitamin D3 (75 +/- 4 nmol/l) than the group with low calcium excretion (57 +/- 4 nmol/l) (p less than 0.002), while the group of patients with intermediate calcium excretion had 25-hydroxyvitamin D3 levels between the other two groups (69 +/- 4 nmol/l). A highly accurate method based on isotope dilution-mass spectrometry was used to assay 25-hydroxyvitamin D3. Of the patients with hypercalciuria (n = 47), seven were classified as hyperabsorbers on the basis of calcium load tests. These patients were found to have even higher serum levels of 25-hydroxyvitamin D3 (108 +/- 10 nmol/l)--significantly higher than that of the hypercalciuric patients as a whole. The above study was carried out in March 1983. In September, the group of patients with high urinary calcium excretion also had significantly higher levels of 25-hydroxyvitamin D3 than the group of patients with low calcium excretion.(ABSTRACT TRUNCATED AT 250 WORDS)