The occurrence of flagellated protozoa in the intestine of man is world-wide and of considerable prevalence. Of recent years there has been considerable attention paid to them as possible causative factors in disease. The organisms have not lent themselves well to thorough experimental investigation, and their common prominence in clinical studies of maladies of obscure origin makes them naturally liable to such suspicion. The fundamental fault, it seems to me, in previously reported clinical studies of "flagellosis" is the lack of control study. It is recognized that proper scientific control of such studies, which do not lend themselves to experimental confirmation, is most difficult. It is with this in mind that the present study has been conducted. An analysis has been made of 1,040 consecutive clinical cases in which gastro-intestinal studies had a prominent place on account of the symptoms displayed, the study being spread over a period of about