We measured immunoreactive endothelin-1 in plasma of a patient with malignant hemangioendothelioma. A 74-year-old female first visited our clinic in July, 1989. Physical examination revealed reddish nodule measured 4 x 5 x 1 cm and erythematous plaque with erosion on her scalp. A biopsy specimen showed extensive proliferation of atypical tumor cells intermingled with a large number of capillary vessels in dermis. Some of tumor cells were positively stained for factor VIII related antigen by PAP method. From these findings we diagnosed malignant hemangioendothelioma and resected the tumor on August 3. The measurement of immunoreactive endothelin-1 in plasma was performed by radio-immunoassay on August 2 (before operation), October 20 and December 27 (after operation). The results were 16.2, 4.3 and 5.6 pg/ml, respectively (normal range: 0.5+/-0.2 pg/ml). Plasma endothelin-1 level may be run parallel to quantity of tumor cells in malignant hemangioendothelioma.