The contact angle of water was measured on surfaces composed of random hydrophilic and hydrophobic patches with typical length scales of 10-100nm. By quenching self-assembled monolayers at various stages of growth, the fractional surface coverage of the hydrophobic patches was varied in the range 0.04-0.97 as determined by atomic force microscopy. The cosine of the contact angle of water cos theta was systematically lower than the prediction of the mean field Cassie equation cos theta(C). The deviation from this prediction cos theta(C) - cos theta had an approximately linear dependence on the total contour length between hydrophobic and hydrophilic patches (a measure of the degree of heterogeneity). The contact angle was insensitive to droplet size, suggesting that line tension effects were minimal. Also, contact angles of hexadecane were in good agreement with the Cassie prediction. We propose two possible explanations for the observed behavior. Long-range (approximately 5 nm) hydrophobic interactions may result in a relatively hydrophobic boundary region around each hydrophobic patch which effectively increases the coverage of the hydrophobic phase altering the equilibrium contact angle. Alternatively, an increased density of "pinning" sites may prevent the contact angle from relaxing to the equilibrium value.