We made a descriptive epidemiological study of patients who presented with isolated macrostomia during a mass screening programme and repair of orofacial cleft deformities in Lagos, Nigeria. Detailed histories of environmental, gestational, and hereditary factors that may contribute to the malformation were taken, and physical examinations and specialist consultations to detect associated congenital malformations were made. Three patients had unilateral macrostomia and in 12 it was bilateral and symmetrical. The mean (SD) age was 8 (2) years, range 3 months to 32 years. There were 6 men and 9 women. Two women and one man had unilateral macrostomia; the two women presented with right sided involvement, while the man had left sided involvement. The commonest associated malformation was a low-set ear (n = 7). Other patients had combinations of congenital malformations. The mean (SD) age of the mothers at conception was 24 (1) years, range 18 to 32 years, while those of the fathers during the periods of conception were 35 (1) years, range 26 to 45. No gestational or environmental factors were detected in the history. One mother with unilateral presentation had a child with bilateral deformities.