BACKGROUND Endoscopic balloon dilation is under investigation as a treatment modality for bile duct stones. It may have an advantage of preserving the sphincter of Oddi function, but little is known about sphincter of Oddi cyclic motility after dilation. METHODS Four dogs with a duodenal cannula underwent sphincter of Oddi dilation and repeated manometry to assess sphincter of Oddi cyclic motility until 3 months after dilation. Histologic changes in the sphincter of Oddi were examined in another group of four dogs. RESULTS Motility index (sum of amplitude of sphincter of Oddi phasic waves counted per minute) and basal pressure decreased on day 3. Sphincter of Oddi amplitude during phase III of the duodenal migrating motor complex tended to be increased on day 3 and decreased to the minimum on day 21. Thereafter, it gradually recovered to baseline. By histology, severe acute inflammation was present in the sphincter of Oddi muscle layer on day 3. However, basal pressure remained significantly low even 3 months after dilation. CONCLUSIONS Sphincter of Oddi amplitude is incompletely reduced on day 3 after balloon dilation. Sphincter of Oddi basal pressure and motility index in the early phase of sphincter of Oddi cyclic motility remain low for at least 3 months after dilation. Further long-term follow-up is necessary to determine whether sphincter of Oddi function is actually preserved.