Objective: To determine whether at the time of HIV-1 seroconversion rapid progressors to AIDS and a low CD4 + count can be distinguished by the clinical presentation of primary HIV-1 infection and serological and immunological characteristics.Design: Prospective cohort study on HIV-1 infection in homosexual men.Setting: The Municipal Health Service, Amsterdam, The Netherlands.Subjects: One hundred and eight men who seroconverted for HIV-1 during follow-up.Main outcome measures: Progression to AIDS and progression to a CD4 + lymphocyte count < 200 × 106/I.Results: Symptomatic primary HIV infection with fever and skin rash, absence of anti-HIV core and transient HIV p24 antigenaemia were independent predictors of progression to AIDS at the time of HIV-1 seroconversion. A low CD4 + count immediately after seroconversion and the calendar year were independent predictors of progression to a low CD4 + count at the time of HIV-1 seroconversion.Conclusions: Even in the earliest stage of HIV-1 infection a small group of individuals at high risk for rapid progression to AIDS can be recognized by the clinical presentation of primary HIV infection, the presence of HIV p24 antigenaemia and the absence of a serological response to HIV core protein.