BACKGROUND: Drug-drug interactions (DDIs) are of great concern, as they are known to be related to adverse drug reactions and hospitalisations. In addition, many DDIs are regarded as predictable and avoidable; therefore, they may be considered as targets for education and interventions. OBJECTIVE: To analyse the relationship between number of dispensed drugs and the probability of potential DDIs among the elderly by using the new Swedish Prescribed Drug Register. METHODS: We analysed data on age, sex and dispensed drugs for people aged > or = 75 years who were registered in the Swedish Prescribed Drug Register from October to December 2005, and constructed a list of current prescriptions for every individual on the arbitrarily chosen date of 31 December 2005. Thereafter, we included those who had at least two dispensed drugs to capture the elderly population at risk of being exposed to DDIs (n = 630 743). The main outcome measures were potentially clinically relevant DDIs (type C), which may require dose adjustment, and potentially serious DDIs (type D), which should be avoided. RESULTS: The prevalence of type C potential DDIs was 26% and of type D potential DDIs 5% in the study population. There was a strong association between number of dispensed drugs and the probability of type C potential DDIs and an even stronger association for type D potential DDIs, after adjustment for age and sex. In addition, the probability of type D potential DDIs decreased with increasing age, and women had a lower probability of type D potential DDIs than men. CONCLUSION: There seems to be a strong relationship between number of dispensed drugs and potential DDIs, especially for potentially serious DDIs, which has implications for the importance of trying to minimise the number of drugs prescribed in the elderly. Our findings that the probability of potentially serious DDIs decreases with increasing age among the elderly and that elderly women have a lower probability of potentially serious DDIs than elderly men need to be verified and investigated by further research.