Background: Acne is one of the most common skin diseases especially in adolescence. Different studies have reported unequal rates of facial acne prevalence in different countries and populations. Only a few cases of acne in the trunk area (back and chest) have been reported in literature. Although our clinical experience shows lower prevalence of truncal acne in comparison with facial acne, a community based study is needed to support this experience.

Methods: A total number of 1001 high school students, selected randomly from 5 out of 20 education-ministry subdivisions of Tehran, were included. In each area two high schools (one for boys and one for girls) with almost 100 students per high school were selected. Demographic data, family history and clinical findings were recorded in the questionnaires. Consensus Conference on Acne Classification was used for acne grading.

Results: One thousand one high school students, 503 girls and 498 boys ,were included. Prevalence of acne was 91.1% for face (95%CI: 83-99%), 93.4% in boys and 88.6% in girls. It was 53.4% for back (95%CI: 46-62.2%), 58.5% in boys and 36.9% in girls. Whereas for chest the prevalence was 36% (95%CI: 27-45%), 34.9% in boys and 36.9% in girls. Mean age of the students with truncal acne was 16.1 years where as 15.9 in others. This difference was significant (P<0.05). Positive family history was higher in students with truncal acne (P<0.001).

Conclusion: Truncal acne is less prevalent than facial acne. Acne on the back is significantly higher in boys than girls (P=0.002). Severe forms of acne in back may be more prevalent in boys. Positive family history can increase the risk of truncal acne.