This investigation examined the formulation of diagnostic hypotheses by general internists and family physicians in response to three patient cases (dyspnea, abdominal pain and syncope). The investigation was conducted in the United States. Physician responses to sequentially presented written clinical information were audiotaped. Each transcribed protocol was scored to enumerate and characterize the hypotheses considered by physicians in each specialty. Results of the analyses of variance of hypothesis measures revealed that internists generated more hypotheses than family physicians and that the internist's hypotheses were more specific and were less likely to be generated by other physicians. In addition, internists tended to consider hypotheses more closely related to the final diagnosis sooner in the case presentation than did family physicians. The findings of increased number, specificity, and uniqueness of hypothesis considered by internists are consistent with previously demonstrated differences in the amount and nature of diagnostic information collected by family physicians and internists.