This research explored whether differentiating patients whose severity of illness worsened, improved, or remained the same over the hospital stay is a good screen for quality of care. The hypothesis was that substandard care is more likely to occur among patients who have worsened. Severity was measured using the Computerized Severity Index (CSI) and MedisGroups in 233 patients who had experienced acute myocardial infarction and 279 who had undergone coronary artery bypass graft who were admitted to four New England hospitals in 1987. Deaths and patients with discharge diagnoses indicating iatrogenic events and complications were oversampled. Potential quality problems were identified through explicit screening criteria applied by nurse researchers and implicit physician reviews. Acute myocardial infarction patients who worsened had higher rates of potential quality problems than other patients (CSI, P = 0.06; MedisGroups, P = 0.01). For the CSI, the 49.4% of patients who worsened captured 70.6% of the potentially substandard care; for MedisGroups, the 35.6% of patients who worsened also encompassed 70.6% of the problematic cases. For coronary artery bypass graft, results varied depending on how severity and quality were defined. The CSI performed better using implicit physician review to identify problematic care (P = 0.00), capturing 76.5% of substandard cases among the 41.6% of patients who worsened. In contrast, MedisGroups did better using explicit quality screens (P = 0.04), grouping 60.5% of the problematic cases among the 47.0% of patients who worsened. After removing in-hospital deaths from consideration, a worsening trajectory was generally associated with a higher fraction of potential quality problems among live discharges. This preliminary study suggests that examining changes in illness severity may be a useful screen for substandard hospital care, but its utility could vary by condition and by how quality problems are defined.