ADAM12 (A disintegrin and metalloprotease) is one of the candidate genes demonstrating susceptibility to osteoarthritis. The purpose of this study was to investigate the relationship between ADAM12-S protein and radiographic knee osteoarthritis (KOA) and its correlation to several bone and cartilage biomarkers. The ADAM12-S protein was measured in 276 subjects (60% women, aged 32-60 years), including 181 individuals with and 95 without radiographic KOA features. The radiographs were obtained from both tibiofemoral (TF) and patellofemoral (PF) joints. The serum levels of ADAM12-S protein were measured by DELFIA1/AutoDELFIA research kit. The ADAM12-S protein was found in detectable ranges in 43 subjects (16 men), without statistical difference between the two genders. In the whole group, the ADAM12-S was related to radiographic KOA grades in TF (P = 0.004) as well in PF joint (P = 0.003). We also found a correlation between ADAM12-S protein and osteophytes in TF and/or PF joints (P = 0.003). No correlations were found between serum levels of S-CTx-I (C-terminal cross-linked telopeptides of type I collagen) or S-PINP (type I procollagen N-terminal propeptide) and ADAM12-S. Similarly, in the whole group, the ADAM12-S protein was not correlated with U-CTx-II (urinary C-telopeptide fragments of type II collagen); however, in the female group, trend to positive correlation between the investigated biomarkers (P = 0.019) was observed. The ADAM12-S protein could be elevated in some KOA cases, and this elevation correlates with the grades of the disease, mostly owning to development of osteophytes. This finding suggests the possible involvement of the ADAM12-S protein in the pathogenesis of KOA.