Six experiments were conducted in different regions of the State of São Paulo to study the effect of N, P and K on the yield of sweet potatoes (Ipomoea batatasLam.) grown on poor "cerrado" soils. Two of these experiments were localized on areas cropped for some years and fertilized with NPK in the previous crops; the others, on areas never fertilized. Potassium increased significantly the yields in all of the experiments. The effect of phosphorus was low in the two experiments conducted on areas previously fertilized but high in the other four localities. Nitrogen increased the yield in only one locality.