A study was carried out comparing the reporting of adverse reactions to nitrofurantoin in Sweden, Holland, and the United Kingdom. The Swedish drug regulatory authorities receive more reports of adverse reactions to nitrofurantoin than to any other drug, and the annual reporting rate is increasing despite a steady fall in the prescription of nitrofurantoin. In the United Kingdom the total number of adverse reaction to nitrofurantoin reported to the Committee on Safety of Medicines has been steadily falling for many years, as has the number of prescription. Lung reactions and liver damage are more commonly reported in Sweden than in the UK, where gastrointestinal symptoms and peripheral neuropathy are more common. Such figures as were available in Holland showed an intermediate picture. The incidences of blood dyscrasias and allergic reactions are similar in the three countries. The causes for such differences are unresolved, but it is disturbing that the incidence and range of adverse reactions to such a common drug as nitrofurantoin reported to the national authorities can differ so much in countries that have close ethnic, dietary, and economic similarities. Thus great caution must be observed when data on adverse drug reactions from the country are extrapolated to another.