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BACKGROUND: Joint and nail involvement in psoriasis is relatively common. In contrast, bony involvement of the terminal phalanx under a psoriatic nail is rare. We report on this psoriatic onycho-pachydermo-periostitis and suggest pathophysiologic mechanisms. OBSERVATIONS: Two new cases are reported and the data are compared with the data from eight similar cases reported in the literature. All subjects presented with similar changes— onychopathy, soft-tissue thickening, and radiologic features consisting of bone erosions and a periosteal reaction of the terminal phalanx. Of the 10 patients, two had no history of psoriasis before a diagnosis of psoriatic onycho-pachydermo-periostitis was made. The inflammation is likely transmitted from the psoriatic nail to the adjacent underlying bone by the same mechanism as in enthesopathies. CONCLUSION: Psoriatic onycho-pachydermo-periostitis should be recognized as a specific entity in the spectrum of psoriatic disease.(Arch Dermatol. 1996;132:176-180)