The leukocyte alklaine phosphatase (LAP) levels were determined in 183 patients with malignant diseases and 71 normal controls. The median LAP scores were 64 units (range 0 to 290) for the patients and 55 (range 2 to 158) for the controls, respectively, and no significant difference could be established. When analyzed according to primary malignancy, only in patients with Hodgkin's disease (n = 14) was the median value higher than normal (p less than 0.001). In patients with distant metastases (n = 48), higher LAP levels were demonstrated (M = 76, range 21 to 290) as compared to patients with no evidence of metastases (M = 53, range 0 to 229), (p less than 0.01). Thus, LAP activity has very limited value in the diagnosis of malignancies. Its elevation in the presence of malignant disease might, however, indicate metastases.