Objectives: To measure patients' expectations of receiving prescriptions and general practitioners' perceptions of these expectations and to determine the factors most closely associated with the decision to prescribe. Design: Questionnaires were completed by patients waiting to see their general practitioners, and by their doctors immediately after the consultations. Setting: Four non-fundholding group practices in southeast London. Subjects: 544 unselected patients consulting 15 general practitioners. Main outcome measures: Doctors' perceptions of patients' expectations; doctors' decisions to prescribe. Results: 67% (354/526) of patients hoped for a prescription; doctors perceived that 56% (305/542) of patients wanted prescriptions; and doctors wrote prescriptions in 59% (321/543) of consultations. Despite the close agreement between patients' hopes and doctors' perceptions, 25% (89/353) of patients hoped for a prescription but did not receive one. In 22% (68/313) of consultations in which prescriptions were written, they were not strictly indicated on purely medical grounds, and in only 66% (202/306) of consultations in which prescriptions were written were they both indicated and hoped for. Doctors' perceptions of patients' expectations were the strongest predictor of the decision to prescribe, but the final regression model also included patients' hopes and ethnic group, and the doctor's feeling of being pressurised. Conclusions: In an area of low prescribing and high expectations the decision to prescribe was closely related to actual and perceived expectations, but the latter was the more significant influence. Evidence that patients' expectations influence general practitioners' prescribing is equivocal; in this study patients' hopes of receiving a prescription exceeded both doctors' perceptions and the level of prescribing Over a quarter of patients who hoped for a prescription did not receive one In a fifth of consultations in which a prescription was written, the prescription was not strictly indicated on purely medical grounds Doctors' perceptions were the strongest determinant of the decision to prescribe Doctors who felt pressurised were less likely to write a prescription if they perceived that the patient wanted one, and if they did write a prescription, it was less likely to be indicated than when the doctor did not feel pressurised