Pachyonychia congenita (PC), a rare autosomal-dominant keratin disorder caused by mutations in keratin genes KRT6A/B, KRT16 or KRT17, is characterized by painful plantar keratoderma and hypertrophic nail dystrophy. Loss-of-function mutations in the filaggrin (FLG) gene underlie the most prevalent skin disorder of cornification, ichthyosis vulgaris (IV), which presents with generalized scaling and is also associated with atopic dermatitis. Recently, FLG mutations have been reported to increase phenotype severity of X-linked ichthyosis and alopecia areata. We report a parent-child trio in which the mother and the son have PC and the father has IV. Both the mother and the son are carriers for the KRT16 mutation p.Leu132Pro. The son, who is much more severely affected than his mother, in addition carries the heterozygous FLG mutation p.R2447X, which was inherited from the father. This observation suggests that coinheritance of mutations in KRT16 and FLG may aggravate the PC phenotype and that FLG could serve as a genetic modifier in PC.