This research study was undertaken to determine prevalence of health misconceptions of twelfth grade students in each of three sizes of public high schools in the state of Colorado. Also, whether prevalence of misconceptions was related to factors of: sex, grade-point average, level of father's education, level of mother's education, father's occupation, and public high school formal health instruction. Procedure involved compilation of harmful health misconceptions in nine areas of health (personal health, nutrition, rest and exercise, care and prevention of disease, habit-forming substances, first aid, consumer health, family living, and mental health) and administration of a Health Opinionnaire to 349 high school seniors. Findings led to the following major conclusions: (1) students subscribed to many health misconceptions, suggesting that secondary education needs to do much in the area of health education, (2) health misconception scores tended to decrease as size of school increased, and (3) percentage of incorrect responses was greatest in areas of consumer health and nutrition and lowest in areas of habit-forming substances and rest and exercise. This suggests a need for health education to sell the public the truth in consumer health knowledge as effectively as they have been sold by advertisers and media sources. (Author/BL)