Calcification of tophi is a well-known secondary phenomenon in gout. Because there are no data available on the nature of the calcium component, light and scanning electron microscopic as well as X-ray microanalysis studies were performed on calcified tophi. In unstained histological slides, urate crystals were detected as negative birefringent needles; after incubation of the slides in distilled water, the urate crystals had disappeared, only positive birefringent calcium pyrophosphate dihydrate (CPPD) crystals of different size remained. An identical result was obtained by scanning electron microscopy. By X-ray microanalysis, peaks for calcium and phosphorus were measured, indicating the presence of CPPD. The occurrence of CPPD crystals in urate tophi as well as in foreign body granulomata indicates that the occurrence of CPPD is not restricted to articular tissues. The biochemical mechanism leading to the appearance of these crystals remains unknown, but it may be assumed that an increase of connective tissue destruction may be responsible for this calcification process.