The Marfan syndrome is a serious heritable connective-tissue disorder characterized primarily by ocular, cardiovascular, and musculoskeletal abnormalities but also involving multiple other tissues and organs of the body. Inherited as an autosomal dominant disorder, the etiology and pathogenesis of the Marfan syndrome are presently unknown. We have documented consistent apparent deficient content of elastin-associated microfibrillar fibers by indirect immunofluorescent (IF) studies of Marfan skin, as well as deficient accumulation of related fibrous materials in cultures of Marfan fibroblasts as compared with normal controls and patients with other heritable disorders of connective tissue. These data have suggested that abnormalities in the microfibrillar component of elastic-fiber systems may have a role in the etiology and pathogenesis of the Marfan syndrome. In the present study, we have analyzed the IF staining patterns of skin and fibroblast cultures from Marfan syndrome patients and normal first-degree relatives in nine Marfan kindreds. Three of these families had at least one affected individual in each of 2 generations, permitting intergenerational comparison of IF patterns. Six kindreds had one or more affected individuals in a single generation, making comparisons between siblings and/or parent-child possible. In all cases, IF abnormalities cosegregated with the Marfan phenotype and all nonaffected family members were normal. Within family groups containing more than one affected individual, the IF staining patterns were similar between affected patients. These data provide further confirmation of consistent and relatively specific deficiency of microfibrillar fibers in Marfan syndrome.