1. The concepts which 100 laymen have concerning cancer, tuberculosis, heart disease, nervousness, insanity, health and disease, masculinity and femininity, were elicited by interview. These are summarized in tables.2. Numerous discrepancies may be found between a layman's concept and the known facts.3. Such discrepancies constitute causes for insecurity through unnecessary anxiety about diseases not found to be present.4. Incomplete or false concepts may be the basis of inadequate protection to a person with the symptoms and signs of serious disease.5. False concepts may cause the usual signs of disease to be distorted.6. False concepts may thus be the basis for psychogenic vulner-ability under certain circumstances and to certairt individuals.7. False concepts may, at times, be as deleteriıus as disease-producing bacteria.8. The study of concepts of disease and conditions has important mental hygiene and public health implications.