Streptococcal infections of oral tissues are mainly seen in young children who experience a variety of upper respiratory tract infections. The disease is characterized by fever, lymphadenopathy, and ulcers on the gingiva, lips, and tonsils. This case report presents an atypical streptococcal infection of the gingiva in an 18-year-old man. The patient was referred to the periodontology department complaining of a 2-month history of gingival enlargement. He had persistent fever (39.5°C) and general malaise for 2 weeks. Intraoral examination revealed extremely inflamed and enlarged gingiva with spontaneous bleeding and suppuration. Based on the otolaryngologic consultation and the hematologic, immunologic, and microbiologic tests, the final diagnosis was an atypical streptococcal gingivitis with chronic adenoid-related mouth breathing and oral hygiene neglect as contributing factors. Treatment consisted of a broad-spectrum antibiotic regimen, supragingival and subgingival debridement, adenoidectomy, and scaling and root planing. A good response to nonsurgical therapy was achieved despite poor patient compliance, and no recurrence of gingival enlargement was observed after 1 year. Streptococcal gingivitis should be included in the differential diagnosis of suppurative gingival enlargements. Furthermore, chronic mouth breathing may initiate and/or contribute to this disease. Keywords: adenoid, gingival inflammation, leukocytosis, mouth breathing, streptococcal gingivitis, suppuration