Minimal residual disease in patients with operable esophageal cancer is frequently missed by current noninvasive tumor staging. Here we applied an immunocytochemical cytokeratin assay that allows identification of individual esophageal carcinoma cells disseminated to bone marrow. Prior to therapy, bone marrow was aspirated from the upper iliac crest of 71 patients with esophageal cancer at various disease stages as well as an age-matched control group of 20 noncarcinoma patients. Tumor cells in cytologic bone marrow preparations were detected with monoclonal antibodies (mAbs) CK2, KL1, and A45-B/B3 to epithelial cytokeratins (CKs) using the alkaline phosphatase antialkaline phosphatase method. CK-positive cells were found in 14 (36.8%) of 38 cancer patients treated with curative intent and 16 (48.5%) of 33 patients with extended disease. The overall frequency of these cells was 1 per 4 x 10(5) to 82 per 4 x 10(5) mononuclear cells with no significant differences between patients at different tumor stages. After a short median follow-up of 9.5 months (3-24 months), 7 of 11 patients who underwent complete surgical resection but had tumor cells in bone marrow presented with tumor relapse compared to 2 of 19 corresponding patients without such cells (p < 0.01). It was concluded that although bone marrow is not a preferential site of overt metastasis of esophageal cancer, the frequent occurrence of isolated tumor cells at this distant site indicates that hematogenous dissemination of viable malignant cells occurs early in tumor progression.