Residential crowding in both U.S. and U.K. samples of 36-month-old children is related concurrently to the Bracken scale, a standard index of early cognitive development skills including letter and color identification, shape recognition, and elementary numeric comprehension. In the U.S. sample, these effects also replicate prospectively. Statistical controls for income, child gender, maternal age, and maternal education are incorporated throughout. In both samples the association between crowding and cognitive development are mediated by maternal responsiveness. Mothers in more crowded homes are less responsive to their children. (Contains 4 tables.)