BACKGROUND: Variations in practice list size are known to be associated with changes in a number of markers of primary care. Few studies have addressed the issue of how single-handed and smaller practices compare with larger group practices and what might be the optimal size of a general practice. AIM: To examine variations in markers of the nature of the care being provided by practices of various size. DESIGN OF STUDY: Practice profile questionnaire survey. SETTING: A randomised sample of general practitioners (GPs) and practices from two inner-London areas, stratified according to practice size and patients attending the practice over a two-week period. METHOD: Average consultation length was calculated over 200 consecutive consultations. A patient survey using the General Practice Assessment Survey instrument was undertaken in each practice. A practice workload survey was carried out over a two-week period. These outcome measures were examined in relation to five measures of practice size based on total list size and the number of doctors providing care. RESULTS: Out of 202 pratices approached, 54 provided analysable datasets. The patient survey response rate was 7247/11,000 (66%). Smaller practices had shorter average consultation lengths and reduced practice performance scores compared with larger practices. The number of patients corrected for the number of doctors providing care was an important predictor of consultation length in group practices. Responders from smaller practices reported improved accessibility of care and receptionist performance, better continuity of care compared with larger practices, and no disadvantage in relation to 10 other dimensions of care. Practices with smaller numbers of patients per doctor had longer average consultation lengths than those with larger numbers of patients per doctor. CONCLUSION: Defining the optimal size of practice is a complex decision in which the views of doctors, patients, and health service managers may be at variance. Some markers of practice performance are related to the total number of patients cared for, but the practice size corrected for the number of available doctors gives a different perspective on the issue. An oversimplistic approach that fails to account for the views of patients as well as health professionals is likely to be disadvantageous to service planning.