Coronavirus-like particles were identified by electronmicroscopy in the feces of homosexual men. The particles banded at a density of 1.21 g/ml after cesium chloride density gradient centrifugation. To determine whether the presence of this virus might be related to clinical symptoms, several patient groups were studied prospectively. In 8 of 16 (50%) homosexual males with acquired immunodeficiency syndrome (AIDS) or unexplained lymphadenopathy syndrome (LAS), coronavirus particles were found. In contrast, such particles were found in none of 18 heterosexual controls and in only 3 of 20 homosexual males without AIDS or LAS. Thus, coronavirus excretion correlated significantly (2c~<0.01) with the clinical diagnosis of AIDS or with syndromes belonging to the AIDS-related complex. In addition, such particles identified in the serum of one patient with LAS and diarrhea suggest invasion and systemic spread of the agent and underline that this virus behaves differently from" common cold" human coronaviruses.