BACKGROUND: To determine the denominator or the 'population at risk' is a problem which has long been encountered in general practice-based epidemiological research. It is important for calculating epidemiological figures. OBJECTIVES: The aim of this article is to demonstrate how in the absence of a patient list, a reliable denominator can be calculated, starting from the number of patients who contacted their GP in the period of one year. Therefore a brief overview will be given from known approaches, then the new approach will be illustrated on a database named Intego, with data from 43 general practices in Belgium. METHODS: The Intego database contains information about patient contacts, diagnoses, laboratory results and drug prescriptions, extracted from the participants' structured electronic medical record system. The number of patients who contacted the practice in a year can be calculated from the Intego data. On the other hand, the percentage of the population that consults a GP during a particular period was obtained from the reimbursement claims data available from the sickness funds. By combining these two datasets, stratified by age, gender and district, a correction factor was calculated. An estimate of the real size of the Intego practice populations was obtained by extrapolating the yearly contact group by this factor. RESULTS: In 2003 according the Intego-register, 64,161 patients contacted their family practice and this correlated with an estimated practice population of 80,094 patients. The absence of the socio-economic status in the estimation is irrelevant in our model of estimating the practice population. CONCLUSION: The availability of a denominator in general practice-based research is essential to calculate epidemiological figures. This method using a correction factor makes it possible to calculate a reliable practice population. A similar approach will probably also be applicable in other European countries.