Although medical insurance claims data provide an increasingly accessible and widely used source of data for health care research, there are few studies about their sensitivity. This study was conducted to investigate the sensitivity of diagnosed cancers in medical insurance claims data. Every case of Kwangju Cancer Registry registered during 1998-1999 was checked for its status in medical insurance claims. The sensitivity of medical insurance claims was expressed as the proportion of cases who were reported as having cancer among cancer registry cases. The sensitivities of Kwangju and nationwide medical insurance claims data for overall cancer were 87.2% and 92.8%, respectively. For cancer sites, the sensitivity of medical insurance claims data was the highest for breast, followed by thyroid, lymphoma and colorectum, and the lowest for pancreas and kidney. Medical insurance claims data would provide reasonably high sensitivity for the detection of cancer, especially if nationwide medical insurance claims are included. Further studies should examine false positive cases to measure other dimensions of accuracy, such as specificity and predictive value.