Background: The spontaneous reporting system is the most efficient warning system of adverse drug reactions (ADRs). Pharmacists can play an important role in the detection and reporting of ADRs. The factors that affect under-reporting among these professionals are unknown in Spain.Objective: To identify the factors that influence community pharmacists’ ADR under-reporting in Navarra, a Northern Spanish region.Methods: A case-control study was conducted on a population of 802 community pharmacists working in Navarra. Cases were pharmacists who had reported at least two ADRs to the region’s drug surveillance unit between 2003 and 2005 and who agreed to participate in the study (18/20; 90%). A random sample of 60 controls was selected from the 762 pharmacists who had not reported any ADR during the same period of time.Results: Factors positively associated with ADR reporting were age, years of work experience as a pharmacist, participation in educational activities related to the detection and resolution of drug-related problems, the habit of detecting ADRs as part of pharmacists’ duties, having the basic knowledge needed to report ADRs, and disagreement with the common belief among healthcare professionals that ’to report an ADR it is necessary to be sure that the reaction is related to the use of a particular drug’. The most frequently mentioned reasons for not reporting ADRs were the ADR is not serious, the ADR is already known, uncertainty concerning the causal relationship between the ADR and the drug, forgetting to report the ADR and a lack of time.Conclusions: Pharmacists’ knowledge, beliefs, behaviour and motivation play an important role in ADR reporting. Under-reporting might be improved through activities focused on modifying such factors.