Subdural electrodes were implanted over the language-dominant left lateral convexity in 45 patients undergoing evaluation for epilepsy surgery. In 29 patients, additional electrodes were placed over the left basal temporal cortex. We identified language areas by using intermittent electrical stimuli applied while patients read aloud. Aphasia was classified as expressive or receptive based on additional testing performed when electrical stimulation elicited reading arrest in the absence of direct excitatory or inhibitory motor effects. Using correlated logistic regression, we noted no statistically significant differences among Broca's, and Wernicke's areas and the basal temporal language area (BTLA) regarding the frequency with which electrical stimulation interfered with language. Speech production deficits, however, occurred significantly more frequently in Broca's than in Wernicke's area (p = 0.012). In contrast, language comprehension deficits occurred with equal frequency when Broca's and Wernicke's areas were stimulated. These results suggest that both Broca's and Wernicke's areas play important roles in language comprehension and that the primarily expressive aphasia of patients with lesions of Broca's area results mainly from the predominant participation of Broca's area in language production.