New techniques of functional imaging with PET have resulted in improved understanding of the organisation of the human brain at systems level. Many studies have concentrated on the organisation of the cerebral cortex, while the cerebellum has been subjected to less intensive investigations. In the motor system the contralateral cerebellum is intimately related to the functions of the primary motor cortex. This has been demonstrated in a number of studies involving simple and more complex motor activations. The relationship has also been demonstrated in pathology. The role of the cerebellum in the generation of tremor has been the subject of some considerable study with the demonstration of functional relationships between the olivary nucleus and the cerebellum as well as between the cerebellum, thalamus and motor cortex in essential and parkinsonian tremor. The cerebellum is also involved in motor learning. Parts of the motor system exhibit physiological adaptation during the repeated performance of a simple motor task. The cerebellum, in particular, shows a decrease in regional blood flow with practice of a motor task despite a similar performance. A number of recent reports suggest that the cerebellum is also involved in cognitive functions and particularly language. Functional neuroimaging has not addressed this problem explicitly; however, studies of severely depressed patients with cognitive impairments have shown an unexpected but interesting result in that impaired cognition but not the state of mind is associated with increases in cerebellar activity. The reason for the paucity of systematic investigations of cerebellar function has been the limited field of view, sensitivity and resolution of PET cameras.(ABSTRACT TRUNCATED AT 250 WORDS)