We studied acoustic startle response and its modulation by prestimulation and by short-term and long-term habituation in 54 autistic patients and 72 normal age-matched controls. The startle response was measured as the amplitude and onset latency of the integrated orbicularis oculi EMG. There were no consistent significant differences between the autistic and control subjects in startle modulation by inhibitory or facilitatory prestimulation, short-term habituation of startle amplitude, long-term habituation of either startle amplitude or latency, or unmodulated startle amplitude. Differences between autistic and control subjects were limited to prolongation of unmodulated startle onset latencies in the autistics in all of the experimental paradigms (significant p = .005 only in the context of short-term habituation) and a statistically significant (p < .05) slower rate of short-term habituation of startle onset latency in the autistic patients, relative to the controls. Results provide only limited support for hypotheses of brainstem pathophysiology and no support for hypotheses of cerebellar pathophysiology in autism.