Objective. To assess the role of the glycoprotein PRG4 in joint lubrication and chondroprotection by measuring friction, stiffness, surface topography, and subsurface histology of the hip joints of Prg4-/- and wild-type (WT) mice. Methods. Friction and elastic modulus were measured in cartilage from the femoral heads of Prg4-/- and WT mice ages 2, 4, 10, and 16 weeks using atomic force microscopy, and the surface microstructure was imaged. Histologic sections of each femoral head were stained and graded. Results. Histologic analysis of the joints of Prg4-/- mice showed an enlarged, fragmented surface layer of variable thickness with Safranin O-positive formations sometimes present, a roughened underlying articular cartilage surface, and a progressive loss of pericellular proteoglycans. Friction was significantly higher on cartilage of Prg4-/- mice at age 16 weeks, but statistically significant differences in friction were not detected at younger ages. The elastic modulus of the cartilage was similar between cartilage surfaces of Prg4-/- and WT mice at young ages, but cartilage of WT mice showed increasing stiffness with age, with significantly higher moduli than cartilage of Prg4-/- mice at older ages. Conclusion. Deletion of the gene Prg4 results in significant structural and biomechanical changes in the articular cartilage with age, some of which are consistent with osteoarthritic degeneration. These findings suggest that PRG4 plays a significant role in preserving normal joint structure and function.