Minimal residual disease in patients with operable pancreatic carcinoma is frequently missed by current noninvasive tumour staging. We applied an immunocytochemical cytokeratin assay that allows identification of individual pancreatic carcinoma cells disseminated to bone marrow. Prior to therapy, bone marrow was aspirated from the upper iliac crest of 48 patients with ductal adenocarcinoma of the pancreas at various disease stages and an age-matched control group of 33 noncarcinoma patients. Tumor cells in cytologic bone marrow preparations were detected with monoclonal antibodies (mAbs) CK2, KL1, and A45-B/B3 to epithelial cytokeratins (CK) using the alkaline phosphatase antialkaline phosphatase method. CK-positive cells were found in 14 (48.4%) of 31 cancer patients treated with curative intent and in 10 (58.8%) of 18 patients with extended disease. The overall frequency of these cells was 1 to 83 per 5x10(5) mononuclear cells with no significant differences between patients at different tumor stages and lymph node involvement. After a median follow-up of 22.8 months (range 3-48 months), 6 (40.0%) of 15 patients who underwent complete surgical resection but had tumor cells in bone marrow presented with distant metastasis and 7 (46.7%) had local relapse compared to none of 12 corresponding patients without such cells (p<0.05). Univariate survival analyses revealed that the presence of CK-positive cells was predictive of reduced overall survival. In conclusion, anticytokeratin mAbs are reliable probes for the immunocytochemical detection of single pancreatic cancer cells disseminated to bone marrow. Thus the described technique may help identify patients with pancreatic cancer and at potentially high risk of early metastatic relapse. The results promise to be of important assistance for determining prognosis and the consequences in therapy of early stage pancreatic cancer.