Primary human chondrocytes, although a promising cell source for cartilage tissue engineering, tend to dedifferentiate and lose their characteristic gene expression and protein production during monolayer expansion. Low O2 tension in vitro culture has been studied to examine its effect on preventing dedifferentiation, and results have been conflicting. In particular, low O 2 expansion effects on human osteoarthritis (OA) chondrocytes are poorly understood. In this study, we expanded chondrocytes collected from patients with osteoarthritis (OA) under normal (21%) and low (5%) O2 tension then continued with micromass culture under 21% O2 for 3 weeks. Our results suggest that low O2 condition may promote chondrogenic characteristic gene expression in monolayer cells. But improved ECM production, which was seen earlier in animal models, may not be relevant to human OA chondrocytes. Further examination of the mechanical properties of the engineered pellets is needed to confirm the effects of low O2 expansion on human OA chondrocytes and their use in cartilage tissue engineering.