We investigated the pharmacokinetics of carboplatin (CBDCA) in two patients with lung cancer on hemodialysis (HD) by measuring the plasma concentrations of free platinum. Blood was sampled from the two at the start and end of HD. One patient with small cell lung cancer (SCLC) was treated with CBDCA and etoposide, and the other with non-small cell lung cancer (NSCLC) was treated with CBDCA and paclitaxel. Plasma concentrations of platinum before the first HD were 6.76 to 8.16 μg/ml. The mean reduction rate of concentration (after HD/before HD) was 16.4 ± 6.1%, which suggested that HD was highly effective in removing CBDCA. From the second HD, plasma concentrations of platinum at the end of HD were lower than those at the start of the next HD. Thus we speculated that CBDCA is stored in the tissues and then flows back into bloodstream after the plasma concentration of platinum is decreased by HD. Adverse events of grades 3 to 4 neutropenia and thrombocytopenia were observed. The monitoring of free plasma concentrations of platinum is helpful for avoiding severe adverse effects in lung cancer patients on HD.