THE cause of stuttering is unknown(1). Failure to develop left-hemispheric dominance for speech is a long-standing theory(1) although others implicate the motor system more broadly(2), often postulating hyperactivity of the right (language nondominant) cerebral hemisphere(3). As knowledge of motor circuitry has advanced(4), theories of stuttering have become more anatomically specific, postulating hyperactivity of premotor cortex, either directly(5) or through connectivity with the thalamus and basal ganglia(6), Alternative theories target the auditory(7) and speech productions(8,9) systems. By contrasting stuttering with fluent speech using positron emission tomography combined with chorus reading to induce fluency, we found support for each of these hypotheses, Stuttering induced widespread over-activations of the motor system in both cerebrum and cerebellum, with right cerebral dominance. Stuttered reading lacked left-lateralized activations of the auditory system, which are thought to support the self-monitoring of speech, and selectively deactivated a frontal-temporal system implicated in speech production, Induced fluency decreased or eliminated the overactivity in most motor areas, and largely reversed the auditory-system underactivations and the deactivation of the speech production system. Thus stuttering is a disorder affecting the multiple neural systems used for speaking.