Adult mice infected with coxsackievirus B3 (CB3) showed a generalized lymphoid involution. This effect was produced by most of the isolates and clones of CB3 and, to a lesser extent, by coxsackievirus B1 but not by other highly pathogenic picornaviruses. While moderate involution of the thymus also occurred in mild CB3 infections, peripheral changes strictly correlated with the severity of disease. The major alteration of the thymus was a massive cortical depletion, whereas the reduction of spleen and lymph node cellularity appeared devoid of selectivity. No histological and ultrastructural signs of CB3 replication in such organs could be detected. Attempts to demonstrate CB3 replication in thymus, spleen, and lymph node cells were unsuccessful. CB3-induced lymphoid atrophy was not prevented by adrenalectomy and was exacerbated by three different immunopotentiating agents. Thus, certain picornaviruses may cause severe stress-independent lymphoid damage in the absence of viral replication in situ. A role for autoreactive mechanisms is suggested.