The concentration of magnesium in serum has been shown to fall to potentially dangerously low levels after several courses of treatment with cis-diamminedichloroplatinum II (cis-platin). The aims of this study were to examine the effects of magnesium supplementation on predicted outcome of treatment, rate of response to treatment and toxicity of treatment. Sixteen patients with testicular cancer were studied in detail over a 14 month period. One patient with an ovarian dysgerminoma was also included in the study. Eight patients were randomised to receive magnesium supplements both intravenous and oral; nine did not. The non-supplemented group showed significantly greater renal tubular damage as assessed by urine N-acetyl-B-D-glucosaminidase (NAG). There was a trend towards a reduction in treatment delays due to neutropenic episodes in the supplemented group, and serum magnesium concentrations remained significantly higher. Neither group showed differences in tumour growth rates or outcome. These results show that magnesium supplements are of considerable benefit and show no harmful effects in patients receiving cis-platin treatment. It is is suggested that magnesium supplements should be a routine part of the treatment regime, and that these should comprise both i.v. supplements during treatment and oral supplements between courses.