OBJECTIVE--To investigate the prognostic significance of symptomatic primary HIV-1 infection. DESIGN--Prospective study of homosexual men seroconverting to HIV in 1985 and 1986. Patients were followed up at least three times yearly with clinical examinations and T cell subset determinations for an average of 7.2 years. SETTING--Research project centred on attenders for treatment and screening for HIV at the Karolinska Institute, Stockholm. SUBJECTS--19 patients presenting with a glandular-fever-like illness associated with seroconversion to HIV and 29 asymptomatic seroconverters. MAIN OUTCOME MEASURES--Progression to Centers for Disease Control and Prevention stage IV disease, CD4 cell count below 200 x 10(6)/l, AIDS, and death from AIDS. RESULTS--Symptomatic seroconverters were significantly more likely to develop Centers for Disease Control and Prevention stage IV disease (95% v 66%), CD4 cell counts below 200 x 10(6)/l (84% v 55%), and AIDS (58% v 28%) and die of AIDS (53% v 7%). CONCLUSION--A glandular-fever-like illness associated with seroconversion to HIV-1 predicts accelerated progression to AIDS and other HIV related diseases.