Severe neonatal Marfan syndrome has features of the Marfan syndrome and congenital contractural arachnodactyly present at birth, along with unique features such as loose, redundant skin and pulmonary emphysema. Since the Marfan syndrome and congenital contractural arachnodactyly are due to mutations in different genes, it has been uncertain whether neonatal Marfan syndrome is due to mutations in the fibrillin gene on chromosome 15 or in another gene. We studied an infant with severe neonatal Marfan syndrome. Dermal fibroblasts were metabolically labeled and found to secret fibrillin inefficiently when compared with control cells. Reverse transcription and amplification of the proband's fibroblast RNA was used to identify a 3-bp insertion between nucleotides 480-481 or 481-482 of the fibrillin cDNA. The insertion maintains the reading frame of the protein and inserts a cysteine between amino acids 160 and 161 in an epidermal growth-factor-like motif of fibrillin. This 3-bp insertion was not found in the fibrillin gene in 70 unrelated, unaffected individuals and 11 unrelated individuals with the Marfan syndrome. We conclude that neonatal Marfan syndrome is the result of mutations in the fibrillin gene on chromosome 15 and is part of the Marfan syndrome spectrum.