Background: Muscles converge or intertweave around the perioral area, and this can be treated with sequential therapy in infants with cleft lip and palate (CLP). The force of perioral muscles has a great influence on maxillary development and morphology. Perioral force in infants with CLP has not been well studied, and accurate and reliable measurement of perioral force in infants remains a challenge. This study aimed to investigate a new way to accurately and reliably measure perioral force in infants with unilateral CLP (UCLP) and explore the change before and after cheiloplasty. Study design: A perioral force measurement system was developed and applied to measure perioral force at labial frenum area and the commissures on both the normal and the cleft sides of four infants with UCLP before and after cheiloplasty. The results were analyzed using the SPSS 19.0 software. Results: The perioral force measurement system appears to produce valid results in infants with UCLP. Before cheiloplasty, the perioral force of labial frenum area was 1.79 +/- 0.94 g/cm(2) and that of commissure on the normal and cleft sides was 5.41 +/- 1.01 g/cm(2) and 3.12 +/- 1.55 g/cm(2), respectively (P < 0.05). After cheiloplasty, perioral force of labial frenum area was 12.73 +/- 3.51 g/cm(2) and that of commissure on the normal and cleft sides was 7.64 +/- 1.64 g/cm(2) and 7.27 +/- 1.89 g/cm(2), respectively (P > 0.05). (C) 2017 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.