OBJECTIVE: To investigate the association between use of non-steroidal anti-inflammatory drugs and serious, acute non-infectious liver injury. DESIGN: Retrospective cohort study, cross over design. SETTING: Health records from provincial database in Saskatchewan, Canada, 1982-6. SUBJECTS: 228,392 adults who contributed 645,456 person years. All were either using or had used non-steroidal anti-inflammatory drugs. MAIN OUTCOME MEASURES: Number and type of prescriptions for non-steroidal anti-inflammatory drugs. Admission to hospital for newly diagnosed acute liver injury. RESULTS: There were 34 admissions to hospital; 16 among subjects currently using non-steroidal anti-inflammatory drugs and 18 among subjects who were not. The incidence rate among current users was 9 per 100,000 person years (95% confidence interval 6 to 15 per 100,000 person years). Subjects currently using non-steroidal anti-inflammatory drugs had twice the risk of newly diagnosed liver injury as those not currently taking these drugs (rate ratio 2.3; 95% confidence interval 1.1 to 4.9) and an excess risk of 5 per 100,000 person years. The age and sex adjusted risk ratio was 1.7 (0.8 to 3.7). The strength of the association increased when only cases with no concomitant use of other hepatotoxic drugs were considered (4.0; 0.9 to 19.0). The rate ratio for people having received one to nine prescriptions was constant. There was no increased risk with long duration of treatment (1.0; 0.3 to 3.5). CONCLUSIONS: There is a small excess risk of serious, acute non-infectious liver injury associated with the use of non-steroidal anti-inflammatory drugs.