During the past year the question of lithium prophylaxis has been the subject of lively debate (Blackwell and Shepherd, 1968; Hullin et al., 1968; Houghton, 1968; Baastrup and Schou, 1968a; Lader, 1968; Sargant, 1968; Saran, 1968; Baastrup and Schou, 1968b; Melia, 1968; Fieve and Platman, 1968; Cole, 1968; Kline, 1968; Kerry, 1968; Editorial, 1968; Berlyne, 1968; Silverman, 1968; Laurell and Ottosson, 1968; Lancet Editorial, 1969; Whybrow, 1969; Melia, 1969; Angst et al., 1969c). This discussion has revealed considerable uncertainty about the design and assessment of prophylactic trials, and there have been misunderstandings on important points.