In 1997 a new prescription system was introduced in Denmark. The pharmacist must now substitute the prescribed drug with a cheaper version either by a generic prescription (G-substitution) or by an original prescription (O-substitution) unless the prescribing doctor indicates that substitution is not allowed in the specific case. The purpose of this study was to obtain the patients' view on the new prescription system and to identify any related problems. The investigation was based on structured interviews. The interview guide was designed as a questionnaire, which was validated and tested before use. The response rate was 82%. The study showed that 84% of the patients were satisfied with the system and 85% of the patients thought that it should continue. Eighty-three percent of the patients had tried another version of the substituted medicine earlier. Out of these, 6% had experienced more side-effects from the substituted medicine, and 10% felt that the substituted medicine had a weaker effect. There was one case of erroneous medical treatment as a consequence of the substitution system. Only few problems such as more side-effects or less effect of the substituted medicine was experienced by the patients. It can be concluded that the patients in general are satisfied with the new prescription system.