Plasma concentrations of three major vitamin D3 metabolites, 25-hydroxyvitamin D3 (25-OHD3), 24R, 25-dihydroxyvitamin D3 (24,25-(OH)2D3), and 1 alpha, 25-dihydroxyvitamin D3 (1,25-(OH)2D3), were determined serially in patients with gynecological malignancies treated by chemotherapy which included cisplatin. While 25-OHD3 and 24,25-(OH)2D3 levels did not change regularly, 1,25-(OH)2D3 decreased significantly (50%) after only one or two courses of the treatment in all cases. In concert with declining 1,25-(OH)2D3, the PTH levels increased two or three times. The results were explained by the impairment by cisplatin of mitochondrial function in proximal tubules in the kidney.