Experimental allergic encephalomyelitis (EAE) is a T cell-mediated autoimmune disease of the central nervous system (CNS) that has served as the principal experimental model for multiple sclerosis (MS). Susceptibility to disease is thought to correlate with the ability to generate a Th1-type cytokine profile in myelin-responsive T cells, whereas T cells producing a Th2 cytokine pattern, in particular IL-4, are thought to be nonencephalitogenic and also to confer protection against a Th1-type response. However, recent studies using a variety of genetically engineered animals in which the genes for Th1-type cytokines and/or their receptors have been inactivated have called into question the Th1-Th2 paradigm in experimental allergic encephalomyelitis. In this report we have addressed the contribution of IL-4 to disease expression by studying two strains of mice, C57BL/6 and BALB/c, in which the gene for IL-4 has been inactivated. The IL-4-deficient C57BL/6 mice, and to a lesser extent the IL-4-deficient BALB/c mice, developed a more severe form of clinical disease, a more extensive pathologic involvement of the spinal cord, and an increased expression of proinflammatory cytokines in the CNS than their wild-type littermates. BALB/c and C57BL/6 mice showed a slightly different cytokine profile in the CNS. Both groups of animals recovered from the acute clinical episode in a time frame that was essentially identical to that found in the wild-type controls. We conclude that IL-4 plays an important role in modulating the severity of the encephalitogenic process, but does not by itself contribute to spontaneous remission from the disease.