QRESEARCH is a large consolidated database derived from the pseudonymised health records of over 24 million patients. The data currently come from approximately 1500 general practices using the EMIS clinical computer system. The practices are spread throughout the UK and include data from patients who are currently registered with the practices as well as historical patients who may have died or left. Historical records extend back to the early 1990's making it one of the largest and richest general practice databases in the world. The data are de-identified at source and patients can opt out.