A retrospective study of 53 cases with a histological diagnosis of Kaposi's sarcoma (KS) reported to The Cancer Registry of Norway during a five year period is presented. Four cases were excluded from the material because further information contradicted the diagnosis of KS. Of the remaining 49 cases, information of treatment received before the development of KS was obtained in 41 (83.7%). Six (14.6%) of the 41 patients developed KS during systemic treatment with corticosteroids, two of the 6 cases also used azathioprine. None of the patients had undergone renal transplantation. One additional patient had received radiotherapy for a malignant lymphoma prior to the development of KS. In a control group of 242 consecutive patients hospitalized because of basal cell carcinoma of the skin, none had used systemic corticosteroid or cytotoxic drugs. Case reports of the 7 patients with KS developing after immunopressive therapy are presented.