A study was made by transmission electron microscopy of tissue specimens (cartilage, meniscus and synovial membrane) taken from 5 knees presenting radiological and anatomical signs of articular chondrocalcinosis and osteoarthritis. It was part of a broader study which included analysis of the same specimens by macroscopy and light microscopy as well as by X-ray diffraction of the mineral deposits. In cartilage and meniscus juxta-cellular calcium pyrophosphate dihydrate (CPPD) crystals of variable sizes were observed in the extracellular organic matrix, independent of the collagen fibrils. They occured mainly in the superficial and middle zones but could also be seen intermingled with the apatite crystals in the cartilage calcified zone. In synovial membrane most of the CPPD crystals were extracellular but some of them could be seen in cytoplasmic phagocytic vacuoles. These observations are in agreement with those in the literature regarding the ultrastructural picture of chondrocalcinosis and support the thesis that the crystals originate in the cartilage and are phagocytized in the synovial membrane. Although the results of the present study do not provide direct evidence of a relationship between chondrocalcinosis and osteoarthritis, the data of the ultrastructural investigation appear nevertheless of great interest as a complement to the data furnished by light microscopy.