OBJECTIVES: To conduct a systematic review to identify studies that describe factors and interventions at primary care practice level that impact on levels of utilisation of unscheduled secondary care. SETTING: Observational studies at primary care practice level. PARTICIPANTS: Studies included people of any age of either sex living in Organisation for Economic Co-operation and Development (OECD) countries with any health condition. PRIMARY AND SECONDARY OUTCOME MEASURES: The primary outcome measure was unscheduled secondary care as measured by emergency department attendance and emergency hospital admissions. RESULTS: 48 papers were identified describing potential influencing features on emergency department visits (n=24 studies) and emergency admissions (n=22 studies). Patient factors associated with both outcomes were increased age, reduced socioeconomic status, lower educational attainment, chronic disease and multimorbidity. Features of primary care affecting unscheduled secondary care were more complex. Being able to see the same healthcare professional reduced unscheduled secondary care. Generally, better access was associated with reduced unscheduled care in the USA. Proximity to healthcare provision influenced patterns of use. Evidence relating to quality of care was limited and mixed. CONCLUSIONS: The majority of research was from different healthcare systems and limited in the extent to which it can inform policy. However, there is evidence that continuity of care is associated with reduced emergency department attendance and emergency hospital admissions.