From time to time in the Middle East, cases were admitted into hospital bearing the diagnosis of Mepacrine Psychosis. This was a label which one hesitated to accept because, for one thing, the cases presented no single clinical type, and for another, they showed, with few exceptions, little or no evidence of symptoms of an organic reaction type such as might be expected in a toxic psychosis. It was, moreover, important, in view of the widespread use of mepacrine both as prophylactic against malaria and in the treatment of it, that the occurrence of a psychosis during the exhibition of mepacrine should not be ascribed to it, except on the irrefutable evidence of a causal connection between the two. Though I have no figures to indicate the incidence of mepacrine psychosis relative to the number of cases of malaria treated with the drug in the M.E.F., I believe the ratio to be very small indeed. Kingsbury mentions 12 cases among several thousand cases of malaria treated with mepacrine; Allen et al. quote Greene as having encountered 2 cases in a series of 750 so treated, in Malay. Hoops had 1 case of mental excitement among 1,207 cases; Briercliffe 15 cases of delirium among some hundred persons taking mepacrine. Field estimated the incidence of mepacrine psychosis to be less than 01 per cent. of cases treated. Gaskill and Fitz-Hugh noted 35 cases of toxic psychosis in 7,064 cases of malaria treated with mepacrine in an army hospital during the recent war; Mergener reports 1 case in 1,000, and quotes Udalagama, who recorded 7 such cases in a series of 644, and Bispham, who noted only 1 case in 7,915 cases that came under his personal supervision, and who up to 1941 found only 35 instances in the literature. Burnham quotes Dove as having noted no case of alarming reactions (presumably including psychotic reaction) in a series of 30,000 cases treated with mepacrine.