OBJECTIVE: This study evaluated the relationship between sibling position and sexual risk based on behavioural and syphilis infection data from sexually transmitted infection (STI) patients in South China. DESIGN: A cross-sectional study examining sexual behaviours and syphilis infection. SETTING: 4 STI clinics in the Pearl River Delta of South China. PARTICIPANTS: 1792 Chinese men and women attending STI clinics. PRIMARY OUTCOME MEASURES: STI history, syphilis infection defined as positive non-treponemal and treponemal tests. RESULTS: Among all clinic patients, 824 (46.3%) were first-born, 354 (19.9%) were middle-born and 602 (33.8%) were final-born. Middle-born individuals had a higher percentage of reported STI history (44.7% compared to 34.7%, p<0.001) and syphilis infection (9.7% compared to 4.9%, p=0.01) among men (n=1163) compared to other sibling positions in bivariate analyses, but not in the final multivariate model. The relationship between sibling position and syphilis was independent of income and education level. There was no trend observed between middle-born position and female sexual risk behaviours (n=626). Higher education was significantly associated with syphilis among women and men in respective multivariate models. CONCLUSIONS: This study suggests that middle-born men in China may have an increased sexual risk compared to other sibling positions. As Chinese family and social structures change, a more thorough understanding of how demographic factors influence sexual risk behaviours is needed.