The Illinois Trauma Registry (ITR) was developed as the principal evaluative tool for the comprehensive set of medical programs known as the Illinois Trauma System. In order to determine the completeness of case reporting to the ITR, a 10 per cent sample of traumatic injuries was drawn at 33 hospitals designated as Trauma Centers. An attempt was then made to link the cases found at the Trauma Centers with those in the ITR; theoretically, all cases found in the Trauma Center should appear in the ITR. Analysis reveals that the ITR is drastically underreported; for all 33 Trauma Centers combined, only 36.9 per cent of the cases appeared in the ITR. On an individual hospital basis, completeness ranged from 0.0 per cent (eight hospitals) to 83.3 per cent. Inclusion in the ITR is significantly associated with the categorization of hospital emergency capability, the method of patient transport to the hospital, the mechanism of injury and subsequent admittance to the intensive care unit. Of particular interest for evaluative studies is the marginally significant difference between the case fatality rate as reported in the ITR (5.0 per cent) and that found in the hospital (2.8 per cent). Guidelines are suggested to improve the development, management and data quality of future registries.