Leukocytosis (WBC counts 10,000/mm3) was detected in 77 out of 252 patients (30%) with ten different types of nonhematological malignancy (NHM) at the time of diagnosis. A full search including serological and bacteriological screening was performed to exclude other possible causes of leukocytosis. Among the different tumors, carcinomas of the lung and colorectum were the most prevalently associated with leukocytosis. Absolute monocytosis was found in 25% of the patients and absolute eosinophilia in only 4.8%. The leukocytosis was attributed mainly to an increase in the mature polymorphonuclears, suggesting a release mechanism of WBC from storage pools by factors secreted or induced by the tumor. Neither the age nor the sex of the patients affected the incidence or magnitude of leukocytosis. However, the presence of metastases was associated with a significantly higher incidence of leukocytosis (p less than 0.05). The associated leukocytosis may be regarded as a poor prognostic sign, and was associated with a significantly (p less than 0.007) shorter survival time. In contrast, absolute lymphocytosis may have a positive effect on the survival time (p = 0.01). Tumor-associated leukocytosis may be an additional tumor-associated marker, of value in assessing and monitoring patients with NHMs.