An ultrastructural study of articular cartilage from five patients with calcium pyrophosphate dihydrate (CPPD) crystal deposition disease was performed. The CPPD crystals, identified by micro X-ray diffraction, were usually found in clusters, located in intercellular areas of the intermediate cartilage layer. The matrix surrounding the clusters either showed a normal morphology or a homogeneous appearance, within which faint cross striations but no distinctly fibrillar outlines could be identified. Another change in the matrix, characterized by an increased electron density and longitudinally fragmented collagen fibres, was also seen. This latter change was generally seen without concomitant CPPD crystal deposition. No specific relationship between the crystals and the collagen fibres or the granular background material of the matrix was encountered.