The reasons for differences in adverse reaction reporting rates between countries are partly methodological, partly due to the ways in which drugs are used and partly due to factors affecting the populations within countries such as disease prevalence, age distribution, genetic differences amongst others. Whilst these factors make international comparisons difficult to interpret, there can be some advantages in the global approach since some of the differences (e.g. a drug used in a special way for a disease seen only in some countries) may provide situations where early signals of drug problems are accentuated. Also the systems used to detect ADR's in one country may have advantages over others though this has yet to be agreed. On the other hand methodological problems arising out of discrepancies in definitions and terminology can give rise to apparent differences between countries and should be obviated by international agreement. There is very little published information on international differences but a careful analysis of the data may give new insights into drug safety and lead to a general improvement in pharmacovigilance methodology.