The C-shaped canal system often is a challenge to optimal cleaning and shaping. This present study was aimed to determine the incidence of fused root patterns, which indicates the presence of a C-shaped canal system, in mandibular second molars of ethnic Chinese in Hong Kong. One thousand clinical and radiographic records of patients in a dental teaching hospital were randomly selected and screened. From the panoramic radiographs, 399 patients with a total of 769 mandibular second molars were suitable for analysis. Of these selected patients, 370 (92%) presented with both 37 and 47, of which 108 (29%) pairs showed the same fused root pattern. Of all 769 second molars examined, 203 (26%) presented with a radiographic Type I morphology, and 68 (9%) and 54 (7%) with Type II and III morphology, respectively. It was concluded that approximately 42% of fused-root mandibular second molars of Hong Kong Chinese patients might be associated with a C-shaped root canal system.