Through Sept 30, 1987, two thousand fifty-nine patients with acquired immunodeficiency syndrome (AIDS) and no recognized risk factors were reported to the Centers for Disease Control. Risk history was incomplete or unobtainable for 921 (45%) of them. Risk factors were ultimately identified for 825 (72%) of the remaining 1138. Another 32 persons (3%) did not meet the case definition for AIDS. Risk factors could not be identified for the remaining 281 patients (25%), despite additional information. Of these, 178 (63%) were interviewed with standard questionnaires; 38% reported sexually transmitted diseases and 34% of the men reported sexual contact with prostitutes. There was no evidence for new transmission modes. Although the proportion of AIDS patients with undetermined risk factors has increased significantly during the past year, the adjusted proportion shows no significant change over time. Thus, follow-up of AIDS patients with no apparent risk factors suggests that modes of transmission for human immunodeficiency virus have remained stable.