Impacted third molars are a common reason for referral to the hospital dental service. Third molar impaction can be complicated by infection. We present two cases of osteomyelitis of the mandible developing secondary to pericoronitis of partially erupted lower third molars. One of the cases reported was recently diagnosed and treated while the other was diagnosed and treated 20 years ago. The most commonly reported pathology associated with impacted lower third molars is pericoronitis. Osteomyelitis of the mandible secondary to pericoronitis is rare. Clinical Relevance: It is helpful if dental practitioners are able to distinguish between the cases of pericoronitis that need emergency referral to hospital and the cases that can be managed in practice and referred to an outpatient clinic.