BACKGROUND: Practice-based research networks are growing and undertaking larger and more complex studies to inform the clinical practice of family physicians. We describe a study that compares clinical behaviors of physicians in the Ambulatory Sentinel Practice Network (ASPN), a large national practice-based research network, with those from the National Ambulatory Medical Care Survey (NAMCS). METHODS: A survey, replicating NAMCS, was conducted among 129 family physician members of ASPN. Nested logistic regression was used to determine which services could predict ASPN membership after adjustment for common and easily observed patient and physician characteristics. RESULTS: Of 20 specific patient services, only 4 were predictive of membership in ASPN. Of these 4, 2 were screening or diagnostic services; ASPN physicians were 1.18 times more likely to obtain a blood pressure measurement and 0.60 times as likely to order a culture for streptococcal pharyngitis. ASPN physicians were 2.30 times more likely to provide family planning counseling and 1.66 times more likely to provide smoking cessation counseling after adjusting for patient smoking status. CONCLUSIONS: We conclude that there are minimal differences in the practice patterns of family physicians participating in a large national practice-based research network and those included in the probability sample of NAMCS. Additional work is needed to examine further those characteristics of the phenomena observed in practice-based research network research that might affect generalizability of results to the larger community of practicing family physicians.