Blood selenium was measured in normal males and females in different age groups, in patients with cancer, and in patients with other diseases. Blood selenium values for 48 normals were 22.9±3.52 µg/100 ml (standard deviation). The values were almost the same in the 18–49 and in the 50–80 age groups for both males and females. Patients with gastrointestinal cancer or metastases to gastrointestinal organs had significantly lower blood selenium values. This was also true of 6 patients with Hodgkin's disease and those with hepatitis and cirrhosis. Patients with carcinoma of the breast, Crohn's disease, certain sarcomas, most patients with rectal carcinoma, and those with several other diseases had normal values of blood selenium. Measurement of blood selenium may help to detect gastrointestinal cancer and Hodgkin's disease. The observation that blood selenium is lower in patients with gastrointestinal cancer is consistent, in general, with results of epidemiologic studies.