AIMS In post-marketing setting, spontaneous reporting by physicians is a mode of surveillance of adverse effects associated with drug use. The objective of this study was to quantitatively assess under-reporting of adverse drug reactions (ADRs) in general practice. METHODS A random sample of 100 general practitioners (GPs) practising in the region of the Bordeaux pharmacovigilance centre were surveyed to obtain data on adverse effects observed. Overall, 81 GPs agreed to record during 3 non-consecutive working days any effect they believed to be associated with drug use. The types of effects, regardless of their seriousness and labelling, and the drugs suspected were characterized and compared to spontaneous reports received from GPs by the Bordeaux pharmacovigilance centre during the reference period. RESULTS The average number of ADRs observed per day per GP was 1.99. The estimate of the under-reporting coefficient (U) was 24,433 (95% confidence interval: 20,702-28,837) which indicates that, as a whole, GPs might be expected to report only 1 out of every 24,433 ADRs to the pharmacovigilance centre. Under-reporting was lowest for serious and unlabelled effects (U = 4610; 95% CI: 2514-8454) and for drugs marketed recently (U = 12,802; 95% CI: 8174-20,050). CONCLUSIONS Adverse effects due to drugs are part of GPs routine activities. According to the observed trend in under-reporting, there appears to be a selection process which indicates that spontaneous reporting in general practice is not conducive to an exhaustive description of the safety profile of a drug. However, our findings are consistent with greater efficacy of spontaneous reporting in detecting serious and unlabelled effect.