PURPOSE The Health Improvement Network (THIN) is a UK healthcare database composed of computerized information from primary care physicians (PCPs). We analyzed the validity of our method for identifying cases of intracerebral hemorrhage (ICH) and subarachnoid hemorrhage (SAH) within THIN and assessed the incidence of these events. METHODS Patients aged 20-89 years were identified and followed until (i) ICH or SAH was detected, (ii) the patient reached 90 years old (iii) death, or (iv) the end of the study. Computerized patient profiles were reviewed manually; those not discarded became potential cases. A validation study was undertaken in 400 computer-detected cases (333 confirmed as potential cases; 67 discarded). PCPs completed a questionnaire to determine the actual incidence of ICH and SAH among these cases. We also assessed the incidence of ICH and SAH in the total cohort. RESULTS A total of 4330 patients with a READ code suggesting hemorrhagic stroke were identified. Computerized profiles with free-text comments were reviewed manually to identify 3633 potential cases. Responses to the PCP questionnaire were received for 306 potential cases and 63 discarded cases (92% response rate); 82% of potential cases were confirmed. Finally, we identified 3137 cases of hemorrhagic stroke. Crude incidence was 15 per 100,000 person-years for ICH and 11 per 100,000 person-years for SAH; the overall incidence increased sharply with age. CONCLUSIONS Computer detection of cases of hemorrhagic stroke in THIN followed by manual review of clinical profiles is a valid method. The incidence of hemorrhagic stroke increases sharply with age.