Pachydermoperiostosis (idiopathic hypertrophic arthropathy) {MIM 167100} is an uncommon disease characterized by unique phenotype (digital clubbing and pachydermia) and distinctive radiographic appearances (periostosis). Two families are reported that, in additional to the typical phenotype and radiographic characteristics of pachydermoperiostosis, show some rare and/or unusual, not yet reported, clinical findings. In the first family, distinctive features were severe progressive arthritis with villonodular involvement of the knees. The clinical course of the disease was much more severe than usually reported. The older brother was disabled at the age of 29 years. In the second family, the clinical history was exceptional, with unique early appearance of clinical signs. Pachydermoperiostosis is usually inherited as a dominant trait, but probable autosomal recessive inheritance has been reported. Also in the present families, autosomal recessive inheritance is likely, possibly explaining the severe clinical course of the disease. Differential diagnosis and the confusing nomenclature of pachydermoperiostosis are discussed.