Abstract The anatomy of the orbicularis oris muscle was studied using histological sections of 18 operative specimens of unilateral cleft lip (14 incomplete and 4 complete). In incomplete clefts the intrinsic part of the orbicularis, located in the vermilion, is simply interrupted without distortion. The extrinsic part, lying higher in the lip, crosses the cleft but is distorted vertically according to the degree of the nasal deformity. In complete clefts the intrinsic bundle ends in the submucosa of the vermilion as in incomplete clefts. The extrinsic bundle is deviated towards the ala nasi on the lateral side. On the medial side, the fibres are rarer and more horizontal. Conclusions are drawn regarding reorientation of the muscle fibres during cheiloplasty.