Cytokeratins are released from carcinoma cells by unclear mechanisms and are commonly used serum tumor markers (TPA, TPS, and CYFRA 21-1). We here report that soluble cytokeratin-18 (CK18) is released from human carcinoma cells during cell death. During necrosis, the cytosolic pool of soluble CK18 was released, whereas apoptosis was associated with significant release of caspase-cleaved CK18 fragments. These results suggested that assessments of different forms of CK18 in patient sera could be used to examine cell death modes. Therefore, CK18 was measured in local venous blood collected during operation of patients with endometrial tumors. In most patient sera, caspase-cleaved fragments constituted a minor fraction of total CK18, suggesting that tumor apoptosis is not the main mechanism for generation of circulating CK18. Monitoring of different CK18 forms in peripheral blood during chemotherapy of prostate cancer patients showed individual differences in the patterns of release. Importantly, several examples were observed where the increase of apoptosis-specific caspase-cleaved CK18 fragments constituted only a minor fraction of the total increase. These results suggest that cell death of epithelially derived tumors can be assessed in patient serum and suggest that tumor apoptosis may not necessarily be the dominating death mode in many tumors in vivo.