Background. Since the turn of the millennium, out-of-hours primary health care in The Netherlands has faced a substantial change from small locum groups towards large GP cooperatives. Improving the quality of care requires evaluation of patient satisfaction. Objective. To develop a reliable postal questionnaire for wide-scale use by patients contacting their out-of-hours GP cooperative and to present the results of a national survey. Methods. Literature review and interviews with both patients and health carers were carried out to identify issues of potential relevance, followed by two postal pilot studies and additional interviews to remove or rephrase items. Finally, postal questionnaires were sent to 14 400 people who contacted one of 24 GP cooperatives in The Netherlands. Results. Overall response was 52.2% for all types of contact. Three scales were identified prior to the field phase and confirmed by principal components analysis: telephone nurse, doctor and organization. Reliability was high, with Cronbach's alphas and intraclass correlation coefficients exceeding 0.70 for all scales. Only items in the organization scale showed clear differences among the participating cooperatives. Respondents receiving telephone advice showed lower levels of satisfaction than respondents with other types of contact ( P < 0.001); centre consultation scored lower than home visit ( P < 0.030 or less for all differences). Conclusion. A reliable measure of patient satisfaction has been developed that can also be used for the comparison of GP cooperatives on an organizational level. Overall satisfaction was high, showing highest levels for home visit and lowest levels for telephone advice.