1. Osteo-articular manifestations in the context of osteoarthropathia psoriatica are characterized by two completely different processes. 2. The intra-capsular synovitis does not differ qualitatively in respect to twelve morphological characteristics from that of rheumatoid arthritis. Only RA-necroses never occur in psoriatic arthritis. 3. The extra-capsular process occurs at the compact as well as the spongy bone of the phalanges. The process shows no sign of a previous or current inflammation or of osteoclast activity. It is characterized by a focal loss of proteoglycan interstitial substance and the exposure of the collagen fiber matrix. 4. This focal proteoglycan loss sets off a process which takes place in four phases: a) Loss of proteoglycan interstitial substance and exposure of the preserved collagen fiber matrix of the bone. b) Deposition of osteoblast chains to the area of the exposed bone and formation of osteoid within the old collagen fiber framework. c) Remodelling of bone defects through filling in of the preserved collagen fiber matrix with newly formed woven bone. d) Reconstruction of this bone into lamellar bone, whereby excessive bone structure ("protuberances") may develop, but final balance of bone remains negative. 5. The described process, which begins with proteoglycan loss, is considered as an enzymatic disturbance, which is causatively connected to the skin process. Scintigraphic examinations indicate that this extra-articular process in the bone, in contrast to arthritis, in part clinically unobserved, is to a high percentage connected to the skin process and is, therefore, probably an integral part of psoriatic disease.