The Anderson-Carr theory of renal-stone formation, based on cadaver studies, postulates the aggregation of calcium at the tips and margins of the renal pyramid. Progressive calcium deposition is followed by the formation of calcium plaques, which may perforate the calyx and form a nidus for further stone growth. This theory has not been demonstrated in vivo. We studied 50 children with conditions leading to nephrocalcinosis with renal sonography. Seven of these had high-resolution CT. Twenty-four positive sonographic examinations were used to study patterns of calcium deposition in the kidney. Nephrocalcinosis was confined to the medulla and was found at the margins of the pyramid, at the fornix, or throughout the entire pyramid. Five children showed calcium plaques in or near the calyx. The sonographic pattern identified appears to provide an in vivo demonstration of the Anderson-Carr progression of renal-stone formation.