BACKGROUND The aim of the study was to determine whether the official numbers of regular general practitioners (GPs) participating in out-of-hours duty have been correct. METHODS Data were collected from the Norwegian labour and welfare organisation's (NAV's) regular GP database. This is a database with information collected from Statistics Norway and the Norwegian Social Science Data Services. RESULTS The study has revealed an error in one of the sources used by the national health authorities to evaluate the regular GP Scheme "Fastlegeordningen". Official figures indicate that the number of regular GPs who participate in out-of-hours duty increased with 5% the first three years after implementation of the regular GP Scheme (2001-2004). New analyses show that the number of regular GPs instead decreased with 12%. CONCLUSIONS Our study uncovers weaknesses with the quality assurance of data used by the health authorities to calculate the work force needed for out-of-hours duty. This misinformation may have prevented action needed to recruit regular GPs to work in the out-of-hours emergency services, as the problem has not been recognised.