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Background: The Health Improvement Network (THIN) database began in 2003. It consists of anonymised records from over 300 general practice computer systems and is likely to be valuable for research, planning and strategic issues in health care, but it is important to establish completeness and accuracy of the data. Aim: To investigate the validity of THIN data for non-melanoma skin cancer (NMSC). We defined NMSC as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Methods: Using Read codes we extracted THIN database records of first-recorded diagnoses of NMSC from 1 January 1996 to 31 December 2003. Searches for SCC were unable to distinguish between skin tumours of this type, and SCC at any other site. From our dataset for BCC, 40 patient records were selected at random, and a questionnaire sent to their corresponding practice, asking if they had been referred to hospital/dermatology clinic, and how the diagnosis of BCC had been confirmed. Results: All the patients in the sample were referred to a hospital or dermatology clinic: 37/40 (93%) had the diagnosis of BCC confirmed, either by a letter from the hospital or a pathology report, a finding that we have reported previously. One patient's diagnosis was confirmed as SCC, and the other two either died or moved away before diagnosis could be confirmed. The 38 patients with diagnoses confirmed were all treated in hospital or dermatology clinic. Conclusions: Data for BCC are sufficiently accurate for research. It is also likely that these data will prove valuable for quality management. It is not possible currently to obtain accurate data for SCC of the skin from the THIN database. This seems not to be a problem with the THIN database itself, but attributable to the Read coding scheme being, in practice, unable to allow differentiation between SCCs of different organs.