We report a young child with a large congenital cervical plexiform neurofibroma and multiple café-aul-ait spots in a generalized distribution who has mosaicism for complete deletion of the NF1 gene. The deletion was demonstrated with intragenic cosmid probes as well as YACs spanning a 700-kb contig including NF1, by two-color FISH with an NF1 and a control probe. Using different intragenic probes, deletion was found in 77-84% of cultured peripheral blood lymphocytes but not in cultured skin fibroblasts. Neither parent has signs of neurofibromatosis type 1 (NF1) or a gene deletion. This is the first report of mosaicism for complete deletion of the NF1 gene. The child did not have typical NF1 or display segmental features of NF1.