A 33-year-old patient, treated for several years with lithium carbonate for a manic depressive disorder, received four courses of combination chemotherapy (bleomycin, etoposide, cisplatin) after the diagnosis of disseminated testicular cancer. Lithium therapy was continued throughout all the courses. Serum and urine lithium concentrations were determined during and between all chemotherapy courses. During the first course a transient 64% decrease in serum lithium concentrations was found. This effect became less pronounced during the consecutive courses. The changes in serum lithium concentrations were without perceptible clinical significance. However, careful monitoring of serum lithium concentrations is mandatory in patients treated with cisplatin-based chemotherapy, as this is accompanied by profound disturbances of lithium pharmacokinetics.