We use the term "occasionally-occurring familial brain tumours" (OFBT) to designate cases in which more than one member of a family presents a brain tumours outside the range of the clinical syndromes in which CNS cancer heredity is well established. We report our experience of OFBT and review the published work on such cases with special reference to some clinicobiological concordances among affected families. On the evidence we present and on that of several genetic and epidemiologico-statistical studies we are inclined to think that there are genetic factors predisposing to the onset of OFBT.