Now that primary care practice-based research networks are known to be feasible, it is important to learn more about the reasons practices participate and withdraw. The Ambulatory Sentinel Practice Network (ASPN) experience presents a special opportunity to study 32 practices that withdrew from the Network since it began in 1982. One hundred percent of these practices responded to a structured telephone survey. The desire to be part of a group doing research (47%) and response to recruitment by an esteemed colleague (28%) were the most important reasons for joining the Network. Changes within the practice (50%), the additional burdens associated with ASPN (22%), and lack of support among practice colleagues and staff (13%) were the most important reasons for withdrawing. Six suggestions were made that could have helped these practices continue in ASPN. Thirteen percent of practices that withdrew rejoined the Network at a later date. Eight years after initiating investigations, 70% of the practices that were ever part of ASPN remained fully involved.