The accuracy of computer held medical information may be of critical importance in patient care, therefore it is important not only to know the error rate in the stored data but also to know the effectiveness of error checking and detection programmes. This paper reports on the errors which were detected in the University of Southampton Primary Medical Care computer system (CLINICS) by checking the consistency between stored data and incoming data. Seven per cent of incoming data had important errors of kinds not normally detected by many medical record systems. The majority were traced either to the registration of new patients or to the doctors failing to pay adequate attention to detail in their record keeping (or to their legibility). They have been subsequently corrected, and it is calculated that the stored data contains less than 1% errors. We suggest ways of improving this; and conclude that certain items are essential to general practice information systems.