A method is described for the imaging and planimetric measurement of the planum temporale (PT) using computer reformations from three-dimensional fast low angle shot magnetic resonance (MR) data. The procedure allows the determination of size and left-right asymmetry of this area on the posterior supratemporal surface, which has been assumed by previous investigators to be a structural correlate of speech lateralization. To establish a consistent method for future studies, special effort was invested in the elaboration of reliable imaging criteria for the borders of the PT. The authors report MR planimetric measurements of the PT in 10 human cadaver brains performed independently and in a "blinded" fashion by two observers. A high degree of interobserver agreement was found, demonstrating the reliability of the MR criteria for PT delineation. The planimetric data correlated with those from subsequent anatomic measurements following section of the same specimens. The left-right ratios of PT size were almost the same with both methods (mean 1.9). Magnetic resonance planimetry of the PT is a noninvasive, reliable, and quantitative procedure to determine gross cerebral asymmetry in the posterior speech area.