Cooperative studies have been designed as a means of obtaining definite answers to significant questions not readily answerable by other means. Such studies should be initiated only if an important problem needs a rapid solution, the study is feasible and likely to answer the problem, and strong and stable leadership are assured. Protocol and operating procedures must be kept as simple as possible. Control of performance at all levels must be built into the structure. Organizational components should include: (a) a Policy or Advisory Board, (b) an Executive or Steering Committee, (c) a Coordinating Center, and (d) data-contributing participants. All of these components must interrelate with each other and with National Heart Institute staff and, through the staff, with scientific review groups and the National Advisory Heart Council. A chart of organization is shown in Figure 1. The most important position is that of Chairman of the Steering Committee; the key component is the Coordinating Center. The performance of the Coordinating Center is continuously dependent on full-time, highly disciplined leadership that must continually maintain active lines of communication with all participants. Failure to achieve this seriously undermines the effectiveness and value of the study. Participation in a cooperative study, with required adherence to a common protocol, can divert some scientists from original research. Conversely, others may be introduced to research methodology through participation. The benefits that can be achieved only through cooperative efforts must be carefully balanced against any adverse effects of encouraging large numbers of investigators to work in cooperative studies. Exceedingly complex value judgments are required of reviewers, who must constantly keep in mind the need to maintain a reasonable balance within the funds appropriated for research. Their deliberations should include an evaluation of technical approaches, organization, biostatistical aspects, duration, and budget. Comprehensive annual progress reports and recall of former consultants could facilitate continuity of review. Free communication between the National Advisory Heart Council and the initial review groups is essential to enhance working relationships and understanding between these two levels of review. Approval of a preliminary study should carry with it a degree of commitment to a major study if feasibility can be demonstrated and adequate methodology developed.(ABSTRACT TRUNCATED AT 400 WORDS)