Twenty-four volcanic rocks from the Central American arc have been analyzed for their Sr and Nd isotopic compositions. On a Nd-Sr isotope correlation diagram, these lavas define an unusual positive trend that correlates strongly with crustal thickness. Nicaraguan lavas, situated on the thinnest crust, have MORB-like Nd but elevated Sr isotopes and provide strong evidence for incorporation of a subducted slab component. Lavas from Costa Rica and Guatemala have lower values of both isotopes and are underlain by crust 30% thicker than that beneath Nicaragua. Isotope correlations with trace elements support the idea that the Central American volcanics are related by two-component mixing of mantle above the subducted slab (including a high 87Sr/86Sr fraction derived from altered oceanic crust) and lower crust (with isotopic values that lie along the mantle array). The correlation of chemical parameters with crustal thickness can be explained if the lower crust acts as a “density filter.” Thus, the least fractionated lavas (Nicaragua) have the highest Sr and Nd isotopic ratios (they have the highest MgO and lowest SiO2 and are the most isotopically similar to the melted mantle wedge). If fractionation is coupled with assimilation and the extent of both is controlled by crustal thickness, then lavas above thick crust will have isotopic values more similar to lower crust because they are the most fractionated.