One hundred fifty cases of endodontic treatment failures were studied clinically, radiographically, and histologically. Fifty-seven percent of the teeth were asymptomatic. Pain alone and/or associated with swelling was present in 21% of the teeth. There was no correlation between the size of periradicular rarefaction and the occurrence or severity of clinical signs and/or symptoms. Stainable bacteria were demonstrated in 69% of the teeth and were present mostly in the canal. The severity of periradicular inflammation was related to presence of stainable bacteria in the canal. Swelling and pain or a draining sinus tract was often associated with stainable bacteria inside the canal. The development of a radicular cyst associated with an endodontically treated tooth that has failed is not necessarily the cause of endodontic treatment failure.