Diaphragmatic defects may be of such magnitude that they preclude repair or reconstruction without the incorporation of a foreign substance either as a replacement or for the support of an otherwise tenuous repair. Such defects are seen in congenital absence of the diaphragm, in eventrations, and huge diaphragmatic hernias, and after en bloc excision for tumor. The success of vascular prostheses in recent years has stimulated the investigation and development of new synthetic materials for the repair of tissue defects in various parts of the body. The recent work of Usher1,2 on the use of a new synthetic material, Marlex, led us to compare this material with 4 other synthetics in the replacement of diaphragmatic defects. The successful use of a given synthetic material for replacement of the diaphragm is governed by its ease of handling, durability, pliability, minimal stimulation of fibrotic reaction, and relative stability in the presence