Congenital mesoblastic nephroma (CMN) is the most common renal tumor of early infancy. It is usually evident at birth as an abdominal mass and is composed of spindle-shaped cells resembling smooth muscle cells and fibroblasts. There is macroscopic and microscopic infiltration of the surrounding kidney and entrapped tubules and glomeruli are common at the edge of the tumor. In this report, we describe a case of CMN associated with hyperreninaemia and hypertension. We examined the tumor from this case and 11 other cases for the presence of renin using a polyclonal antibody to human kidney renin. Hypertension was present in three of four additional cases for which records were available. Immunoreactive renin was present in ten of the 12 cases studied. In all of these cases, intense staining was present in vessels within the areas of the trapped cortex. In seven cases, renin was identified in the walls of vessels within the tumor itself without identifiable adjacent cortical structures. These findings indicate that CMN may often be associated with primary reninism in early infancy.