The author developed a theory of developmental collaborative diagnosis for small groups of adult learners, constructed a procedural model, and identified possible advantages of such diagnosis. Data were gathered from the fields of adult education, diagnosis, and social psychology, from consultations with adult education professors, and from personal experience. Findings included the following: (1) developmental collaborative diagnosis is the group process of investigating symptoms in order to identify educational problems and needs; (2) underlying principles are need, equilibrium, self preservation, self enhancement, and cooperation, with individual and social behavior combined in a single theoretical system; (3) the principles of freedom of speech, shared program development, active participation, pragmatism, relationship of means and ends, and the importance of the individual are also relevant; (4) when the procedural model is utilized, learners are encouraged to accept personal responsibility for diagnosing and satisfying their real educational needs, cooperative program planning is promoted, and obstacles to effective group learning are discovered and corrected. (author/ly)