During 1975 twenty-six bereaved spouses took part in a detailed prospective investigation of the effects of severe stress on the immune system. T and B cell numbers and function, and hormone concentrations were studied approximately 2 weeks after bereavement and 6 weeks thereafter. The response to phytohaemagglutinin was significantly depressed in the bereaved group on the second occasion, as was the response to concanavalin A at 6 weeks. There was no difference in T and B cell numbers, protein concentrations, the presence of autoantibodies and delayed hypersensitivity, and in cortisol, prolactin, growth hormone, and thyroid hormone assays between the bereaved group and the controls. This is the first time severe psychological stress has been shown to produce a measurable abnormality in immune function which is not obviously caused by hormonal changes.