The present prospective pilot study investigated putative drug interactions in primary health care. Among 110 outpatients, 50 were treated with more than one drug (maximum = 10). Among these 50 prescription forms, 21 concerned at least one drug interaction, with a total of 47 different drug interactions. Most of them (94 per cent) were explained by a pharmacodynamic mechanism. They were classified as 'requiring use precautions' (64 per cent), 'to be taken into account' (25 per cent) or 'inadvisable' (11 per cent). None of them was 'contraindicated'. Most often the drugs involved were diuretics, converting enzyme inhibitors or non-steroidal antiinflammatory drugs. Drug interactions, which concern around one in five prescription forms, remain an important topic in Clinical Pharmacology.