A paradigmatic case in which anticancer chemotherapy could paradoxically lead to immune restoration is reported. A patient with hairy cell leukemia was unsuccessfully treated with alpha-interferon. Treatment had to be withdrawn because of unusual toxicity and therapy with cytotoxic drugs had to be administered. Not only did the anticancer agents produce a complete remission of the disease, but the immunological profile of the patient improved. This case report raises the problem of the interference of chemotherapy with the immune system in cancer patients.