Medication record cards were kept for 1,366 patients over a three-year period at a neighbourhood pharmacy in north-west London. Eighty-six potential adverse drug reactions were detected. In 53 cases the general practitioner changed the prescription after being contacted by the pharmacist. In a further 15 cases advice intended to reduce the likelihood of an adverse drug reaction was given to the patients by the pharmacist. Seventy-six errors on prescriptions were also detected; these were mainly unintended changes in dose or strength of medication. Pharmacists could have a useful role to play in monitoring for potential drug reactions in general practice. Closer collaboration between the two professions would be of mutual benefit.