During development, cells monitor and adjust their rates of accumulation to produce organs of predetermined size. We show here that central nervous system–specific deletion of the essential adherens junction gene, α E-catenin, causes abnormal activation of the hedgehog pathway, resulting in shortening of the cell cycle, decreased apoptosis, and cortical hyperplasia. We propose that αE-catenin connects cell-density–dependent adherens junctions with the developmental hedgehog pathway and that this connection may provide a negative feedback loop controlling the size of developing cerebral cortex.