Background Observational studies have shown differences in process and outcome between the consultations of primary care physicians whose average consultation lengths differ. These differences may be due to self selection. Objectives To assess the effectiveness and efficiency of interventions to alter the length of primary care physicians' consultations. Search strategy The following electronic databases were searched: Cochrane Effective Practice and Organisation of Care Group (EPOC) Specialised Register (October 2002); CENTRAL (The Cochrane Library June 2003); MEDLINE (1966 to October 2002);EMBASE (1981 to October 2002); NHS National Research Register (June 2003). The search strategies combined subject terms for 'general practice', 'consultation' and 'length' with methodological filters. The register search and the Medline search were updated in August 2007. Selection criteria Randomised controlled trials (RCTs) and controlled clinical trials (CCTs) of interventions to alter the length of primary care physicians' consultations. Data collection and analysis Data were extracted independently by two authors using agreed criteria. Disagreements were resolved by discussion. Where data were missing attempts were made to contact authors. Given the heterogeneity of studies meta-analysis was not attempted, and results are presented as a narrative summary. Main results Seven articles describing five UK trials met the inclusion criteria. All tested short term changes in the consultation time allocated to each patient and most had methodological weaknesses, particularly due to non-random allocation of patients. Altering appointment length resulted in modest changes in average length of consultation. There were no consistent differences in problem recognition, examination, prescribing, referral or investigation rates. There was some evidence that blood pressure was checked and smoking discussed more often when more time was available. None of the interventions were associated with differences in patient satisfaction. No trials examined efficiency. Authors' conclusions The findings of this review do not provide sufficient evidence to support or resist a policy of altering the lengths of primary care physicians' consultations. Further trials are needed that focus on health outcomes and cost effectiveness.