OBJECTIVE: The aims of this study were (1) to develop and evaluate a new method for investigating personal drug formularies in general practice and (2) to test the hypothesis that there is a difference between personal drug formularies for incident and ongoing drug use. METHODS: In 2002, we studied prescribing patterns of beta-blockers, calcium channel blockers, angiotensin converting enzyme (ACE) inhibitors, angiotensin-II antagonists and statins in the County of Funen, Denmark (180 general practices, 472,000 inhabitants). For each practice, we distinguished between an incident drug formulary consisting of prescriptions for new users and an ongoing drug formulary including prescriptions for patients already in treatment. Prescription data were retrieved from the Odense University Pharmacoepidemiologic Database (OPED). Four different formulary measures were evaluated and used for comparing incident and ongoing drug use. RESULTS: General practitioners' (GPs') incident drug formularies comprised significantly fewer drugs than their ongoing drug formularies for all drug groups except angiotensin-II antagonists. The difference in the total number of drugs used was between 1.8 and 3.3. We found differences between 0.5 and 1.6 analogues in the DU 90% (number of analogues accounting for 90% of the prescribed volume measured in defined daily doses) segment and the formulary selectivity index between 0.05 and 0.12. The preference for the most prescribed analogue was 9-18% higher among incident patients. The formulary selectivity index was highly correlated with the other formulary measures and quantified both range and skewed distribution of drug choice. CONCLUSION: Analysing GPs' prescriptions to incident patients is a simple and inexpensive method for studying their own current personal drug formularies. GPs choose within a narrow range of analogues for incident patients.