1. The nature and physiological action of Vitamin A are discussed. 2. A resume' of previous clinical work on Vitamin A deficiency is given and Xerophthalmia is established as pathognomonic of Vitamin A deficiency. 3. The incidence of xerophthalmia in a selected group of men is worked out, and is correlated with the incidence of certain diseases and with their general physique. 4. Positive correlation is found between xerophthalmia andlow weight, chronic malaria, enlarged tonsils and cervical adenitis, and acne vulgaris. 5. Negative correlation is found between xerophthalmia and a high state of physical fitness. 6. No correlation is found between xerophthalmia and height, trachoma, otitis media, rheumatic heart disease, seborrhoea and endemic goitre.