This study examined the relationship between the severity of acute human immunodeficiency virus type 1 (HIV-1) illness and disease progression and death. The population included 218 patients with acute HIV-1 illness and 41 asymptomatic patients who underwent HIV-1 seroconversion; the patients were followed up prospectively. We analyzed progression to Centers for Disease Control and Prevention clinical categories B and C (AIDS-defining conditions) and death according to an additive clinical score (CS) based on six predictive clinical features at the time of acute HIV-1 infection. Compared with patients with a CS of 0 (asymptomatic patients), those with a CS of 3-4 and 5-6 had faster progression to category B disease (adjusted hazard ratio [HR], 1.39; 95% confidence interval [CI], 1.01-1.92; and HR, 1.80; 95% CI, 1.34-2.40; respectively); those with a CS of 5-6 had faster progression to category C disease (HR, 1.37; 95% CI, 1.01-1.89) and death (HR, 2.05; 95% CI, 1.27-3.32). Thus, the number of symptoms and signs at the time of acute HIV-1 illness affects disease progression and survival, even in symptomatic patients who have undergone seroconversion.