1. A systematic collection of the clinical findings in fifty patients with pyrophosphate synovitis,among some 300 patients with obscure disorder of the knee, has been made over a period of eighteen months. The numbers of men and women were equal, the mean age being seventy years (range thirty-seven to ninety), and the mean age at the onset of symptoms fifty-nine years.

2. A difference in the clinical picture between the sexes was found. In men an acute synovitis predominated, in women chronic joint complaints.

3. A high incidence of accompanying disease was found, but none had a significant relationship to the arthropathy, although the high frequency (20 per cent) of synovitis following an acute severe illness of some other kind was striking.

4. No specific radiological sign except for calcifications in articular cartilage and menisci was found in these patients, and no relationship to osteoarthrosis could be established.

5. The great variability of symptoms and the surprisingly high incidence attracts attention to pyrophosphate synovitis as a cause of joint symptoms, especially in elderly patients.