The purpose of the study was to assess patients' acceptability of the substitution of brand-name drugs for generic drugs in the primary health care prescribing practices for chronic conditions. A prospective randomised multicentre study was conducted over a 12-month period in which patients taking medications for chronic disorders received an educational intervention on generic drugs at the time they attended different general practices in the city of Barcelona (Spain) for repeat prescribing. Twenty-seven public primary care centres were randomised to the intervention group (eight centres) or the control group (19 centres). Of 4620 patients in the intervention group that received verbal information and handout materials on advantages and disadvantages of generic equivalents and brand-name drugs, 98.9% of them agreed to receive a generic formulation. The primary care centre and the class of drug were associated with statistically significant differences in the percentage of acceptance of generic drugs. In the overall population, generic prescribing in the intervention practices increased to 5.9 as compared with 2.8% in controls. In summary, individual educational intervention in patients with repeat prescribing resulted in a high rate of generic acceptability. The intervention might stimulate the practitioner's motivation, behaviour and knowledge of generic forms.