This data set contains morphometric measures of the lagoons of the Atlantic Coast. The goal was to obtain a measure of the complexity of the marshes that filled the lagoons behind the barrier islands. ABSTRACT The width, depth, marsh cover, and marsh-water interfaces were recorded for the lagoons along the 2000 km of coast between Long Island, New York and Miami, Florida. Eigenvectors of these variables for 134 sites (cases) were calculated and analyzed to identify the characteristic variations of these morphometric attributes. Three modes of variation account for 88% of Ihe variance of the original data: the dominant mode contrasts wide, complex lagoons and narrow, simple lagoons. The second contrasts wide, simple with narrow, complex lagoons. A third mode contrasts wide, shallow, complex with narrow, deep lagoons with few marsh-water intersects. The first mode is correlated geographically with variations in the steepness and curvature of the inner portion of the continental shelf. Using variations in the morphometric attributes of the lagoon-marsh system and the fronting islands on the ocean side, the Atlantic coast barrier islands, lagoons, and marshes are classified into three regions and eight sub-regions. The concept of barrier island ''ensembles'' along the Atlantic coast is reviewed in terms of the island-lagoon marsh system and their covariation with offshore bathymetry. The concept of these ensembles is strongly supported.