Acute HIV-1 illness presents a wide range of clinical manifestations. We assessed a classification and data reduction of clinical features among 218 patients with acute HIV-1 infection enrolled in four prospective seroincidence studies. Factor analysis allows the definition of eight factors based on groups of symptoms and signs: gastrointestinal transit disturbances, weight loss/abdominal pain, lymphadenopathy, myalgia/arthralgia, neurologic features, constitutional and mucocutaneous features, oral candidiasis, and anorexia/pharyngitis. These groups reflected the main target systems involved at the time of acute HIV-1 disease. Grouping of symptoms and signs based on groups of patients is potentially more informative than observations made on individual patients. It might facilitate diagnosis, suggest pathogenic mechanisms and reduce the number of variables for characterizing acute HIV-1 illness.