Exact role of the inflammation in osteoarthritis is still unclear, but it is thought to originate from synovitis due to micro-crystals or breakdown products of the cartilage. To determine the effect of CD30+ T lymphocytes on the development of osteoarthritis by comparing the lesion depth and synovial CD30+ count in patients with chondral lesions undergoing knee joint arthroscopy. A total of 79 patients with chondral lesions detected during arthroscopy were categorized in 4 different groups based on chondral lesion classification. CD30+ lymphocyte counts were calculated using flow cytometry on synovial fluid samples obtained at the time of initial entrance into the joint and compared between the groups. In addition, biopsy samples obtained from the suprapatellar bursa were stained for histologic examination to identify existence of CD30+ lymphocytes in the synovium. Although there were no significant differences between the first 3 groups in terms of synovial fluid CD30+ lymphocyte counts, patients in Group IV had significantly higher counts (6.2 8 [2.48] vs 2.51 [1.84], 2.97 [2.40], and 3.80 [2.07], respectively; P < 0.05). Except for a single patient with a Grade III chondral lesion, there were no cases of CD30 positivity in synovial tissue. Also there was a correlation between CD30 levels and chondral lesion depth when controlled for age. Our results indicate higher CD30+ lymphocyte counts in patients with modified Outerbridge Grade IV chondral lesions than in other groups. The origin of the CD30+ lymphocytes may not be the synovial tissue per se. Thus, it was hypothesized that the injured chondral tissues and the associated subchondral structures might have been the source of CD30+ lymphocytes with a possible influence on the development of osteoarthritis.