The paper deals with the problem of homicidal behaviour with psychotic motivation, draws attention to the difficulty of prediction in this respect. Two case histories from the authors' own observation--one patient with psychotic disease of the schizophrenic type (paraphrenia) and the behaviour of one female patient with manifestations of depressive psychosis who committed this serious crime--indicate the difficulty of psychiatric prognoses. Although an aggressive deed in a paranoid patient is not an exception the way of its implementation unexpected. Less common and quite unexpected was the behaviour of the female patient with involutional depression which provides evidence that these patients are not only dangerous to themselves.