A pseudo-tumoral cerebellar infarction developed in a 48-year-old man over the course of several days. External drainage improved the patients condition, and a subsequent operation showed a type I Arnold-Chiari malformation with cerebellar amygdalae descending to the level of C2. The infarcted postero-external part of the cerebellar lobe and the left amygdala were resected. The left postero-inferior cerebellar artery was thrombosed between the inferior border of the malformation and the posterior arch of C2. Almost total clinical recovery occurred. The Arnold-Chiari malformation had been clinically silent until the present infarction. A low dysraphia with a dermal sinus, sacral dehiscence and a low cord were present, but had caused no clinical disorders. The genetic origin of the dysraphism was confirmed by the presence of lumbosacral myelomeningocele with hydrocephalus in a relative of the patient. The relationship between postero-inferior cerebellar artery thrombosis and an Arnold-Chiari malformation is discussed: a causal relationship, already suggested in previous reports, is likely in the present case.