Objectives: To examine the effect of patients' expectations for medication and doctors' perceptions of patients' expectations on prescribing when patients present with new conditions. Design: Questionnaire study of practitioners and patients. Setting: General practice in Newcastle, Australia. Subjects: 22 non-randomly selected general practitioners and 336 of their patients with a newly diagnosed medical condition. Main outcome measures: Prescription of medication and expectation of it. Results: Medication was prescribed for 169 (50%) patients. After controlling for the presenting condition, patients who expected medication were nearly three times more likely to receive medication (odds ratio=2.9, 95% confidence interval 1.3 to 6.3). When the general practitioner thought the patient expected medication the patient was 10 times more likely to receive it (odds ratio=10.1, 5.3 to 19.6). A significant association existed between patients' expectation and doctors' perception of patients' expectation (χ2=52.0, df=4, P=0.001). For all categories of patient expectation, however, patients were more likely to receive medication when the practitioner judged the patient to want medication than when the practitioner ascribed no expectation to the patient. Conclusions: Although patients brought expectations to the consultation regarding medication, the doctors' opinions about their expectations were the strongest determinants of prescribing. This study showed that patients who expected medications were three times more likely to be prescribed medicines for new conditions If the general practitioner thought that the patient expected medication, patients were ten times more likely to be prescribed medication Although patients brought expectations to the consultation regarding medication, it was the doctors' opinions about patients' expectations that were the strongest determinants of prescribing With the increasing promotion of rational prescribing, practitioners need to be aware of these influences on prescribing