Medullary nephrocalcinosis (MNC) is usually a bilateral process with symmetric involvement of both kidneys. Asymmetric medullary nephrocalcinosis has been previously reported in the literature, but has not been well illustrated or explained. We report the sonographic findings in two pediatric patients with hypercalcemia. In both patients an unrelated unilateral renal abnormality, (renal vein thrombosis in one and obstructive hydronephrosis in the other) prevented the development of MNC in the affected kidney, probably by decreasing the glomerular filtration rate and/or altering the renal tubular function.