The psychosocial aspects of intestinal bypass of intestinal bypass surgery for massive obesity are considered from the standpoint of their implications for therapeutic decisions. The author's experience with a prospective study of 29 bypass patients, reported in 1974, is summarized. Response to weight loss included improvement in activity levels, mood, self-esteem, interpersonal and vocational effectiveness, and a trend toward more normal eating behavior. Notable changes included decreased use of denial as a coping style, and loss of a self-reinforcing sense of entrapment, resignation, and ineffectiveness associated with massive obesity. Psychiatric illness, when it occurred, did not appear to be the direct result of surgery or weight loss. The available literature in the field is reviewed and compared with these results. While there is much support for the impression of substantial psychosocial benefit following weight loss, most reports place greater emphasis on the psychosocial morbidity and psychiatric hazards associated with bypass surgery. Problems encountered in evaluating these reports are discussed, including the importance of the length of the follow-up interval, and the need to describe the interaction between psychosocial morbidity, the patient's psychosocial base-line, and the somatic status. A plea is made against premature utilization of psychosocial screening criteria while knowledge is still so limited, and tentative guidelines for clinical practice and research are proposed.