The possible relationship between military deployment and the subsequent increase in cancer rates has been prominent since the Vietnam War. The objective of this study was to investigate whether any form of cancer was increased among veterans deployed to the Persian Gulf in the 1991 conflict. This study matched data from central cancer registries in the District of Columbia and New Jersey with the records for 1.4 million Gulf War era veterans, i.e., 621,902 veterans who arrived in the Persian Gulf before March 1, 1991, and 746,248 non-Gulf veterans. Using a proportional incidence ratio, testicular cancer was found to be the only significantly increased malignancy among deployed Persian Gulf War veterans. The increase became apparent 2 to 3 years after the Persian Gulf War and peaked 4 to 5 years afterward. Our data and those of investigators studying Vietnam veterans suggest that testicular cancer may be related to military deployment.