Skeletal dysplasias represent in vivo models of genetic defects. Achondrogenesis type II (Langer-Saldino), caused by a genetic defect in the major cartilage matrix protein, collagen type II, is a rare and severe skeletal dysplasia. It comprises a severe derangement of the fetal growth plate cartilage with subsequent ossification defects. In this study, we analyzed the matrix composition and cell differentiation pattern in 3 relatives with achondrogenesis type II. Most strikingly we found a strongly reduced collagen type II and moderately reduced aggrecan proteoglycan content in the dysplastic cartilage matrix. Type II collagen is, at least to some extent, replaced by collagens type I III, and VI. Ultrastructural analysis of the dysplastic cartilage matrix demonstrated a distended rER (rough endoplasmic reticulum), which is typical for this condition and most likely related to improper processing and retention of genetically altered type II collagen. Immunostaining for type IIA and X collagens suggest a severe delay in chondrocyte maturation. Thus, the genetic defect in the present cases leads most likely to a severe retention of collagen type II in the rER and, therefore, a strongly reduced collagen deposition and replacement by other interstitial collagens. However, the latter are less efficient in binding aggrecan proteoglycans in the dysplastic cartilage matrix. Additionally, a delay in chondrocyte maturation appears to be important in achondrogenesis type II.