The lethal chondrodysplasias are a complex group of diseases of which little is known. Three disorders have been well characterized; achondrogenesis, thanatophoric dwarfism, and the lethal chondrodysplasias with polydactyly. Pathological studies of epiphyseal cartilage confirm the separateness of these diseases but further work is necessary to specify the limits of variations. In achondrogenesis, relatively minor forms differ greatly from the usual description. In the chondrodysplasias with polydactyly, skeletal involvement appears to vary from one case to another. Furthermore, we must pursue the genetic studies in thanatophoric dwarfism where the exact mode of inheritance is known.