Asymmetry of the planum temporale, a language-related intrasylvian area on the superior temporal gyrus, is the most remarkable anatomical left-right asymmetry of the human brain. The in vivo application of magnetic resonance morphometry in 52 healthy volunteers (26 dextrals and 26 sinistrals) revealed that planum temporale asymmetry is correlated with hand dominance. Left-handers had a significantly lesser degree of leftward planum temporale asymmetry than right-handers. Thus, a structural-functional relation exists in cerebral asymmetry. The correlation is likely to reflect language representation. Because familial sinistrality influenced the anatomical pattern in left-handers and planum temporale asymmetry is already present in the newborn, prenatal factors must play an important role in the development of functional laterality.