PURPOSE: To assess the usefulness of the talocalcaneal C sign in the diagnosis of subtalar coalition, as seen on lateral radiographs of the ankle. MATERIALS AND METHODS: Lateral radiographs of 12 ankles in 11 patients were included in this study. Twelve subtalar coalitions were confirmed by surgery (n=6), or by CT and/or MR (n=6). The presence of the talocal-caneal C sign on lateral ankle radiographs was determined. RESULTS: The C sign was continuous in six feet and interrupted in the remaining six. Subtalar coalition occurred simultaneously in the middle and posterior subtalar joints in two cases, the posterior subtalar joint only in six, and in the middle subtalar joint only in four. In six cases confirmed at surgery, subtalar coalitions consisted of both synostosis and non-osseous fusion (synchondrosis and/or syndesmosis) and in one case of middle subtalar coalition, there was a bony bridge. The remaining six cases, confirmed at CT or MRI, involved both synostosis and non-osseous fusion (n=1) or non-osseous fusion only (n=5). CONCLUSION: In the diagnosis of subtalar coalition, the talocalcaneal C sign, seen on lateral radiographs of the ankle, is a useful indicator.