Estimates of the incidence of hospitalized injuries based on hospital discharge data are inconsistent because of variations in 1) the definition of injury and 2) the criteria for excluding repeat admissions for the same injury event. Using 1983 data from the Uniform Hospital Discharge Data Set for the state of Rhode Island, the authors demonstrate the effects on injury incidence rates associated with the various definitions and exclusion criteria used in previous studies. The overall injury rate (11.9/1,000 population) was substantially reduced (rate difference, 1.7; 95% confidence interval 1.2-2.1) when adverse effects or complications of medical and surgical care ("medical injuries"), not usually defined as injuries by injury researchers, were excluded. Estimates of the incidence of "true" injury hospitalizations (excluding medical injuries, late effects, and complications of care) ranged from 9.9/1,000, when repeat admissions identified as transfers from another acute facility were excluded, to 7.2/1,000, when repeat admissions identified as elective admissions were excluded. Marked variability in incidence rate estimates by age, sex, nature of injury, and state of residence of victim was also noted. The addition to hospital discharge data sets of a separate variable to identify readmissions for the same problem would result in more valid incidence estimates for injury research and surveillance.