Objectives: To compare the predictors of self-reported medicine underuse due to cost across countries with different pharmaceutical subsidy systems and co-payments. Methods:We analysed data from a 2007 survey of adults in Australia, Canada, Germany, the Netherlands, New Zealand (NZ), the United Kingdom (UK) and the United States (US). The predictors of underuse were calculated separately for each country using multivariate poisson regression. Results: Reports of underuse due to cost varied from 3% in the Netherlands to 20% in the US. In Australia, Canada, NZ, the UK and the US, cost-related underuse was predicted by high out-of-pocket costs (RR range 2.0-4.6), below average income (RR range 1.9-3.1), and younger age (RR range 3.9-16.4). In all countries except Australia and the UK, history of depression was associated with cost-related underuse (RR range 1.2-4.1). In Australia, Canada, Germany, the UK and the US lack of patient involvement in treatment decisions was associated with cost-related underuse (RR range 1.2-1.4). In Australia, Canada and NZ, indigenous persons more commonly reported underuse due to cost (RR range 2.1-2.9). Conclusions: Cost-related underuse of medicines was least commonly reported in countries with the lowest outof-pocket costs, the Netherlands and the UK. Countries with reduced co-payments or cost ceilings for low income patients showed the least disparity in rates of underuse between income groups. Despite differences in health insurance systems in these countries, age, ethnicity, depression, and involvement with treatment decisions were consistently predictive of underuse. There are opportunities for policy makers and clinicians to support medicine use in vulnerable groups.