A four-fold (P less than 0.001) mean increase in iron levels was found in 18 patients (a total of 36 courses of therapy) with ovarian cancer at the end of a 5-day course of cisplatin (40 mg/m2 per day every 4-5 weeks). The kinetics of these modifications began very early (24-48 h after initiation of therapy): they reached their maximum on the 4th-5th day, coinciding with the last drug administration, and basal levels were recovered after the 10th day. A subsequent eight-fold average increase (P less than 0.001) in ferritin serum levels, beginning 2 days after the iron changes, was observed, but showed a slower regression (after the 15th day). Reticulocyte counts were lowered (P less than 0.001) with the same time-course of the iron increases, but returned to pretreatment levels within 2 weeks. Total bilirubin and serum glutamate-pyruvate transaminase showed significantly delayed increases compared with iron. The results are in keeping with a reduced iron utilization by the erythroid precursors, but other mechanisms cannot be excluded. There is no statistical correlation between the early iron increases and the subsequent hemoglobin nadir values.