The clinical symptoms and signs were assessed in 20 consecutive patients developing infection with the human immunodeficiency virus (HIV). All were male homosexuals and all presented with a glandular-fever-like illness. Changes in laboratory values were compared with findings in 40 HIV negative male homosexual controls. In the 10 patients for whom date of exposure to the virus could be established the incubation period was 11-28 days (median 14). One or two days after the sudden onset of fever patients developed sore throat, lymphadenopathy, rash, lethargy, coated tongue, tonsillar hypertrophy, dry cough, headache, myalgia, conjunctivitis, vomiting, night sweats, nausea, diarrhoea, and palatal enanthema. Twelve patients had painful, shallow ulcers in the mouth or on the genitals or anus or as manifested by oesophageal symptoms; these ulcers may have been the site of entry of the virus. During the first week after the onset of symptoms mild leucopenia, thrombocytopenia, and increased numbers of banded neutrophils were detected (p less than 0.0005). The mean duration of acute illness was 12.7 days (range 5-44). All patients remained healthy during a mean follow up period of 2.5 years. Heightened awareness of the typical clinical picture in patients developing primary HIV infection will alert the physician at an early stage and so aid prompt diagnosis and help contain the epidemic spread of AIDS.