The present study investigated the involvement of the human cerebellum in the habituation of the acoustic startle response using PET. The startle response was elicited in seven young, healthy subjects by a tone presented via headphones. Startle responses were recorded from the right sternocleidomastoid muscle. Regional cerebral blood flow (rCBF) was assessed in nine scans and one startle stimulus was applied during each scan. The reduction of size of the sternocleidomastoid muscle response was correlated with changes in rCBF during the ongoing process of startle response habituation. A significant decrease of rCBF was found in the medial cerebellum. These data are consistent with an involvement of the medial parts of the human cerebellum in non-associative learning as proposed by previous animal studies.