Trismus is a firm closing of the jaw due to tonic spasm of the muscles of mastication from disease or the motor branch of the trigeminal nerve. Trismus may be produced by a variety of reasons such as dental abscess, trauma, following mandibular block with local anesthesia, as a result of radiation to the facial muscles, and patients after chemotherapy. A case of a referral of a six-year-old boy to a dentist from an ENT due to severe limitation in jaw opening is presented. Intraoral examination and panoramic radiograph demonstrated no signs of infection and/or other pathology. After a diagnosis of trismus was made, due to his icteric appearance, the general fatigue and loss of appetite in the last few days, palpated and sensitive lymph nodes in the submandibular and cervical regions, the child was referred for a complete blood count and sedimentation rate. The laboratory and clinical findings resulted in the diagnosis of acute lymphoblastic leukemia (ALL). Dental and oral manifestations of ALL are discussed, and the trismus may be explained by an intensive infiltration of leukemic cells into the deep portion of the contracting muscles of the face. This case emphasizes the importance of physical examination and independent judgement made by dentists, even when patients are referred to them by other members of the medical communities.