Papillon‐Lefèvre syndrome (PLS) is a rare autosomal recessive disease that involves severe periodontitis and hyperkeratosis of the hand palms and foot soles. Recently it was found that PLS patients carry loss‐of‐function mutations in the gene encoding cathepsin C (CTSC). In the present study we have analyzed the CTSC gene in two unrelated families with PLS. In the first non‐consanguineous family, mutation analysis revealed the previously reported c.815G>C/p.R272P mutation. The second consanguineous family displayed a c.1213C>A mutation which resulted in the novel mutation p.H405N and is the first mutation described in the active site of the enzyme. The PLS patients had, next to the absence of cathepsin C activity in polymorphonuclear leukocytes (PMNs), no activity of the three serine proteinases elastase, cathepsin G and proteinase 3. Serine proteinases are supposed to be important in both the innate and adaptive immune systems. Their absence in PLS patients could explain the inadequate defense to periodontal infection. © 2004 Wiley‐Liss, Inc.