Three families are described in which each of the mothers took trimethadione during pregnancy. From a comparison of siblings in each family and of others exposed to trimethadione in utero, a specific phenotype is delineated. Features included in the fetal trimethadione syndrome are developmental delay, speech difficulty, V-shaped eyebrows, epicanthus, low-set ears with anteriorly folded helix, palatal anomaly, and irregular teeth. Additional anomalies in some of the patients include intrauterine growth retardation, short stature, microcephaly, cardiac anomaly, ocular anomaly, hypospadias, inguinal hernia, and simian creases.