Treatment by prefrontal leucotomy, though undoubtedly of value, still rests upon an insufficiently scientific basis. Its anatomical and physiological effects are being increasingly discovered, but many of its psychological effects are still unpredictable and obscure. Its effect on intelligence, on immediate memory, and on simple learning ability are now well known, but with these we are not concerned. Rather, we have attempted to assess the effect of leucotomy on “creative ability.” By this phrase we shall refer to that ability by which the ideational content of past experience is utilized in a situation to provide concepts and actions not directly evoked by the new situation, but elaborated by the patient's more complex cerebral reactions to the situation; The ability is worthy of examination, since the more intellectual the work the more is it evident. The architect, for instance, needs more creative ability than the bricklayer, the novelist more than the amanuensis, and the Royal Academician more than the house painter.