A study of urban pharmacies in Guatemala and in Chiapas, southern Mexico, was undertaken to analyse the scale of the inadequate drug advice provided, and to identify the contributing factors. The estimate of the scale of the problem was based on the results of 969 approaches to 191 pharmacies by 'magic clients' (i.e. researchers pretending to be looking for treatment for relatives who had one of three 'tracer' diseases), interviews with 4469 pharmacy users as they left the same 191 pharmacies, and a comparison of the quality of advice offered by public and private pharmacies in Guatemala (based on exit interviews with 150 users). The contributing factors were explored using a provider survey (interviews with 166 pharmacy supervisors and 371 drug vendors), an in-depth study analysing large-chain and independent Mexican pharmacies, and a review of the national drug policies in both countries.