A dramatic increase in the number of adverse reactions to nitrofurantoin initiated a survey of the reports to the Swedish Adverse Drug Reaction Committee 1966-1976. Pulmonary reactions constituted about half of these reports: 398 cases with acute and 49 with chronic reactions. Three-quarters of the patients were hospitalized in connection with the reaction. Six reactions were fatal. The clinical picture and the laboratory findings differed considerably between the acute and chronic forms, although the age and sex distribution was very similar. About 85% of the patients were women. The median age was 59 years in the acute and 68 years in the chronic group. The acute pulmonary reactions carry the characteristics of an allergic reaction but we suggest that the chronic pulmonary and liver reactions may be caused by a toxic mechanism. Chronic reactions do not follow upon acute reactions, nor do acute reactions predispose to chronic ones. Early recognition of the reactions and prompt withdrawal of the drug are essential in both forms.