This study is a comparative analysis of the prevalence, absolute number and aggregation status of bone marrow micro-metastases (BMM) between breast (n=234) and gastric (n=102) cancer patients based on a standardized number of 1 X 10(6) bone marrow-derived cells per patient. Additionally, expression of the epithelial cell adhesion molecule E-cadherin was analyzed on disseminated tumor cells. A positive BMM status was demonstrated in 88/234 breast and 45/102 gastric cancer patients. The presence of CK18+ cells positively correlated with parameters of advanced tumor progression in breast, but not in gastric cancer. Interestingly, 25.2% of the node-negative patients already had micrometastatic cells in the bone marrow at diagnosis. Regarding the absolute number of CK18+ cells and the frequency of CK18+ cell clusters, no significant difference was found between the 2 tumor types. However, clusters consisting of more than 10 CK18+ cells (type II clusters) were present exclusively in breast cancer patients. Additionally, co-expression of CK18 and E-cadherin was detectable in 15/21 micrometastases-positive breast but in only 1/9 gastric cancer patients. While prevalence of micrometastatic cells in bone marrow is discussed as an early indicator for systemic disease, aggregation status and a certain antigen profile might be indicative for site-specific differences in the manifestation pattern of solid metastases.