We studied serum concentrations of calcium, phosphate, intact parathyroid hormone (PTH) and vitamin D3 metabolites in 41 patients with nephropathia epidemica. Thirty-four of the 41 patients had a mild to moderate, mostly nonoliguric acute renal failure (ARF). Hypocalcemia developed in relation to the severity of renal failure, and parathyroid gland response to hypocalcemia was normal. The serum concentration of 1,25-dihydroxyvitamin D3 was lower than normal in patients who developed ARF. Serum phosphate was the most important factor in regulating the serum 1,25(OH)2D3 level, though only mild phosphate retention was seen in the patients. We observed normal or slightly elevated serum phosphate, hypocalcemia accompanied by elevated PTH levels and a decreased serum concentration of 1,25(OH)2D3 in patients with ARF caused by nephropathia epidemica.