Twenty patients with idiopathic extracapillary glomerulonephritis and 16 patients with other forms of crescentic glomerulonephritis were examined by light and electron microscopy. Crescents were found to be composed of several types of cells which can be divided roughly into two groups: light and dark cells. Most of the crescent cells are apparently derived from the pariental epithelial cells of the Bowman's capsule, but at least in some cases podocytes participate in the cresent formation. Breaks in the glomerular capillary basement membrane were frequently seen (in 11 of 15 patients with idiopathic disease and in 4 of 15 other patients). Such breaks may significantly contribute to the leakage of fibrinogen and red blood cells into the Bowman's space and thus accelerate crescent formation. Proliferation of the epithelial cells appears to be related to precipitation of fibrin, though it is uncertain whether the actual stimulus is provided by the fibrin or by a coprecipitate of some normal or abnormal serum protein. Some of the proliferated cells acquire features of fibroblasts and are presumably responsible for laying down of collagen in the crescent, while other cells demonstrate phagocytic properties. These metaplastic changes of the epithelial cells are briefly discussed.