Abstract  Hospital tumor registries may permit efficient identification of relatively large numbers of patients for clinical and health services research. This study investigates the feasibility and cost of using hospital tumor registry data for identifying cancer patients with particular clinical characteristics, corresponding to the protocol entry criteria of four randomly sampled Phase III clinical treatment trials for cancer. We screened tumor registry data on 717 patients diagnosed with cancer in 1997 and 1998 who had been identified and abstracted by the registry of a large academic medical center by April, 1999; and we abstracted the medical records of the 122 patients who passed the registry screen. For each clinical profile, the registry screen eliminated a substantial fraction of patients who did not meet the relevant criteria. Of the patients identified from the tumor registry as potential matches, 41% matched the relevant clinical profile based on detailed medical records review. The cost and effort of the registry-based method were substantially lower than would have been necessary if we had reviewed medical records without the registry screen, suggesting that tumor registry data can be a relatively efficient tool for identifying research subjects.