The length of the cochlear turns was measured in 9 pairs of temporal bones from age-matched male and female individuals (1 day to 76 years old), using a computer-aided three-dimensional reconstruction and measurement method. The mean cochlear length was significantly longer (Two-way analysis of variance, F = 31.87, d.f. = 1, p less than 0.01) in males (37.1 +/- 1.6 mm) than in females (32.3 +/- 1.8 mm), whereas it did not vary with postnatal age in either sex. Sexually dimorphic cochlear length may pose a new issue in auditory physiology in man. The lack of postnatal elongation also indicates that length of the cochlea becomes close to its maximum during fetal life.