To assess reasons to avoid drug use and perceived harmfulness of marijuana and alcohol, data were obtained from 2,926 7th, 9th, and 11th grade students in Ventura County, California in 1985. The five reasons to avoid drug use were addiction, punishment, loss of friends, disappoint parents, and disappoint self. Student responses were compared across sex, grade, ethnicity, and school type and were contrasted with the frequency of actual use of alcohol, cannabis, cocaine, hypnotics, stimulants, and psychedelics. Avoidance reasons were only moderately related to harmfulness, whereas harmfulness was more negatively related to general drug use than the avoidance reasons factor. More females endorsed all five reasons for avoiding drugs than males. Additionally, girls indicated higher mean levels of support for harmfulness items than males. There was a general trend for more students in the lower grades to endorse reasons to avoid drugs than students in the higher grades. Significantly more students enrolled in regular school reported fear of punishment, loss of friends, and disappointing parents as reasons to avoid drugs than continuation school students. Consistently, more Asians endorsed reasons to avoid drugs than other ethnic groups. Blacks, Hispanics, and American Indians showed the least amount of support for the reasons. 36 references, 8 tables, and 1 figure (Author abstract modified)