Our aim was to determine the incidence of synostoses in the bones of the lower limbs in patients with multiple cartilaginous exostosis (MCE) and use the available imaging to suggest the cause and mechanism of its development. Radiographs of the lower legs of 21 patients with MCE were reviewed. With the intention of demonstrating the exact site and extent of synostoses and other bone deformities, such as bone pressure atrophy or erosions in five patients, 8 proximal and 6 distal tibiofibular joints were examined by CT scans. No synostoses were present in 11 patients and 10 patients had 1 to 4 synostoses. Of these synostoses, 14 were localized below the knee joint and 9 above the ankle joint. A growing osteochondroma arising from tibia or fibula can cause an erosion in the contagious surface of the neighbouring bone. If facing osteochondromata are present in both bones and show an interlocking growth at abutting parts, on osseous fusion can take place with formation of a synostosis in the proximal or distal tibiofibular joint region. In adult patients with MCE and abundant osteochondromata synostoses between the neighbouring bones of the lower legs are common findings; they are always caused by coalescence of "kissing" osteochondromata.