In countries with fee for service billing and direct access to specialist care the general practitioner does not know key figures of the population he serves. As the role of general practice in health-service-research, quality assurance activities and the construction of sentinel-networks is becoming more important, valid methods to determine the denominator are needed. Results from five practices suggest that yearly contact groups should be used as a common denominator for general practice in our health care system. For certain groups of patients (e. g. the elderly > 60 yrs.) even quarterly contact groups can give sufficient information to forecast the yearly contact group.