A 41-year-old man was provided with a jacket crown after a root end resection of a molar. Four months later, cortical destruction of the ulnar diaphysis with swelling and pain appeared in his forearm. No microorganism could be grown from an intraoperative tissue specimen, but bacterial 16S rRNA genes were detected by broad-range PCR, revealing Porphyromonas gingivalis as the causative agent of osteomyelitis.