OBJECTIVES: Primary care patient databases are increasingly used within many different research areas.  These databases reflect everyday care provided to patients in a sample of practices.  Thus, when used for research, information on generalisability to the general population is important.  This study compared chronic condition prevalences from a UK primary care database, THIN, with national figures.  METHODS: The study population was selected from THIN Data.  The National Health Service (NHS) Quality and Outcomes Framework (QOF, a national standard used for the measurement of primary care outcomes for payment) disease/drug codelists and rules were used to derive crude prevalence of chronic diseases.  Prevalence and 95% confidence intervals (CIs) were derived as of February 14, 2007, March 14, 2007, and April 1, 2007 from practices in Scotland/Northern Ireland, England, and Wales, respectively, to match QOF methodology.  National QOF 2006/2007 prevalence data were used for comparison.      RESULTS: THIN population included 2,783,593 people.  The atrial fibrillation crude prevalence was in THIN 1.42%(±0.01%) versus 1.31% nationally. For asthma these figures were 5.95%(±0.01%) versus 5.79%, cancer 0.85%(±0.003%) versus 0.91%, coronary heart disease  3.89%(±0.01%) versus 3.67%, chronic kidney disease 2.48%(±0.02%) versus 2.33%, chronic obstructive pulmonary disease 1.57%(±0.01%) versus 1.48%, dementia 0.47%(±0.003%) versus 0.42%, diabetes 3.52%(±0.01%) versus 3.65%, epilepsy 0.63%(±0.002%) versus 0.62%, heart failure 0.88%(±0.004%) versus 0.80%, hypertension 12.67%(±0.03%) versus 12.56%, learning disability 0.30%(±0.002%) versus 0.27%, mental health 0.74%(±0.004%) versus 0.72%, obesity 8.29%(±0.04%) versus 7.50%, palliative care 0.11%(±0.002%) versus 0.09%, stroke/transient ischaemic attack 1.89%(±0.01%) versus 1.65%, hypothyroidism 2.74%(±0.01%) versus 2.63%.        CONCLUSIONS: THIN and national crude prevalences were close but higher in THIN except for cancer and diabetes.  Differences are likely to be due to factors not controlled for such as demographics, socioeconomics, practice infrastructure, and community.