Objectives: We previously reported a cluster of cholangiocarcinoma patients among proof-printing workers who were exposed to 1,2-DCP for a long term. The present study was conducted to evaluate blood parameters in these proof-printing workers during and after exposure. Methods: Health examination records during employment and after retirement were obtained for ten cholangiocarcinoma patients to analyze their blood parameters. The patients and/or their relatives were also interviewed about lifestyle and occupational history. Results: All study patients were exposed to 1,2-DCP for 6–17 years. Red blood cells, hemoglobin, hematocrit, total cholesterol, triglycerides, and fasting plasma glucose were within the standard ranges for almost all patients, but the γ -glutamyl transpeptidase ( γ -GTP) levels exceeded the standard range during 1,2-DCP exposure for six patients. Two of the six patients were diagnosed with cholangiocarcinoma during 1,2-DCP exposure, and the other four patients were diagnosed 1–9 years after termination of exposure. The remaining four patients had γ -GTP levels within the standard range during 1,2-DCP exposure, but had increased γ -GTP levels thereafter, and were diagnosed with cholangiocarcinoma 4–10 years after termination of exposure. Aspartate aminotransferase and alanine aminotransferase levels started to increase following the increase in γ -GTP levels. Conclusions: Workers exposed to 1,2-DCP should be provided with periodic health examinations during and after exposure. In the examination, even small increases in γ -GTP levels should be considered a signal of early development of cholangiocarcinoma.(J Occup Health 2014; 56: 279-284)