The micrometastatic spread of tumour cells is usually missed by conventional diagnostic techniques, although this spread largely determines the prognosis of patients with primary epithelial cancers. By use of the monoclonal antibody, CK2, to epithelial cytokeratin component number 18 (CK18), individual disseminated carcinoma cells present in bone marrow of cancer patients can now be identified. In the present study, this approach has been applied to patients with virginal stage C adenocarcinoma of the prostate. Double-sided aspirates of iliac bone marrow from 24 of 44 evaluable patients (54.4%) exhibited between one and 38 CK18-positive cells per sample of 2 x 10(6) mononuclear cells. In 13 of these 24 positive patients, CK-positive cells were only detected in one of the two aspirates analysed. There was no statistically significant correlation between this finding and established risk factors, such as the volume and histological grade of the primary tumour or the concentration of prostate specific antigen and prostatic acid phosphatase in serum. The follow-up time is too short to provide meaningful data on the prognostic significance of isolated CK18-positive cells in bone marrow, which, however, has been recently demonstrated in other types of primary epithelial cancers. In conclusion, the presence of prostatic tumour cells in bone marrow might be interpreted as an indicator of the metastatic capacity of an individual primary tumour. The immunocytochemical detection of these cells may, therefore, be useful for increasing the precision of current tumour staging, and to monitor minimal residual cancer in an individual patient.