One model to explain the relationship between patterning and growth during development posits that growth is regulated by the slope of morphogen gradients. The Decapentaplegic (DPP) morphogen controls growth in the Drosophila wing, but the slope of the DPP activity gradient has not been shown to influence growth. By employing a method for spatial, temporal, and quantitative control over gene expression, we show that the juxtaposition of cells perceiving different levels of DPP signaling is essential for medial-wing-cell proliferation and can be sufficient to promote the proliferation of cells throughout the wing. Either activation or inhibition of the DPP pathway in clones at levels distinct from those in surrounding cells stimulates nonautonomous cell proliferation. Conversely, uniform activation of the DPP pathway inhibits cell proliferation in medial wing cells. Our observations provide a direct demonstration that the slope of a morphogen gradient regulates growth during development.