INTRODUCTION: The lesions that compromise the labial commissures can originate microstomias ( reduction of the size of the mouth ), with important aesthetic and functional impairments ( feeding, speech and oral hygiene ). They are more frequent among children in the preschool and school age. Most of the cases are acquired, having as main etiology the electric, thermal and chemical burns, being very rare the congenital causes. These deformities generally need surgical treatment, with a careful postoperative accompaniment. CASE REPORT: We shall relate the case of a 36 year old male patient with a severe microstomia after chemical burn with caustic soda that occurred when he was 2 (two) years old. It is noteworthy, that he lived so long with the microstomia without ever having sought medical assistance, making this case a very peculiar situation. He related having little lateral drooling, difficulty in feeding, speech and oral hygiene. What is interesting, is that the patient has never complained of any toothache, whatsoever. On physical examination of the oral region, he was noted to have a severe scar contracture in the region of both the oral commissures, with perioral bilateral fibrosis and difficulty of facial expression (smile). Intraoral examination was difficult to perform because of the limited mouth opening (approximately 25 mm in horizontal and vertical direction). There was normal mobility of the tongue, with hard positioning and hygiene of the teeth. (Figures 1 and 2). The reconstruction of the oral commissures was performed through the Converse-Kazanjian's technique (1959). The position of the neocommissures was based through a vertical line that was positioned on the level of the pupils (Figure 3). The principle of the technique is to create lateral, superior and inferior mucosal flaps, that are advanced and sutured to the skin edges, without tension, to form the new vermilion border ( See details in illustration 1; Figures 4, 5, 6 ). The patient's postoperative course was unremarkable. He was instructed to have liquid diet in the first fifteen postoperatory days, physical exercises (mouth opening) to minimize relapse and local wound care. Six months later he showed significant correction of lateral drooling, commissure competence when the mouth is closed, improvement in feeding, speech and mouth opening (45 mm in the horizontal direction and 35mm in the vertical direction) and acceptable aesthetic result (Figures 7, 8). DISCUSSION: The microstomias are due to perioral cicatricial contractures that are formed usually after the burns, which can lead to aesthetic and functional impairments. Several procedures have been described for the reconstruction of the labial commissures, which can include zetaplasties, skin grafts, commissurotomies and the composed local, nasolabial and tongue flaps. The Converse-Kazanjian's technique ( Described by Dieffenbach's and modified by Conserve ) was used due to an easiness in execution (could be done with local anesthesia ) and good postoperatory results both in the aesthetic and functional point of view. We agree with the former descriptions of Converse that the main factor for the appearance of postoperative relapses are the size and thickness of the lateral mucosal flap, that should not be tense when sutured to the skin edges, avoiding the appearance of bulged tissues and scars, postcontracture relapses and irregularities in the mucocutaneous junction of the lips. CONCLUSION: The surgical procedures used in the treatment of microstomias are in the majority of cases suitable and of good results, however, far to reach a widely satisfactory commissure, both in the aesthetic and functional point of view. We learn with this case that once again, the greatest strides in microstomia have been in prevention. Similar splinting devices have been fabricated in an attempt to prevent the contracture of the commissure. A good orientation, education and measures of prevention are fundamental in order to reduce the number of responsible factors that can lead to the deformities of the oral commissures.