We investigated cerebral processing of audiovisual speech stimuli in humans using functional magnetic resonance imaging (fMRI). Ten healthy volunteers were scanned with a 'clustered volume acquisition' paradigm at 3 T during observation of phonetically matching (e.g., visual and acoustic /y/) and conflicting (e.g., visual /a/ and acoustic /y/) audiovisual vowels. Both stimuli activated the sensory-specific auditory and visual cortices, along with the superior temporal, inferior frontal (Broca's area), premotor, and visual-parietal regions bilaterally. Phonetically conflicting vowels, contrasted with matching ones, specifically increased activity in Broca's area. Activity during phonetically matching stimuli, contrasted with conflicting ones, was not enhanced in any brain region. We suggest that the increased activity in Broca's area reflects processing of conflicting visual and acoustic phonetic inputs in partly disparate neuron populations. On the other hand, matching acoustic and visual inputs would converge on the same neurons.