In 2000, syphilis mass treatment using oral azithromycin was delivered to at-risk British Columbians during a sex trade-related outbreak. The initiative included education, counselling and referral. This cross-sectional, observational study examines knowledge, attitudes and self-reported behaviour after one year among mass treatment participants compared with eligible non-participants. Participants self-reported positive changes: reduction in sexual partners overall (P=0.001) and for sex workers (P<0.01), decrease in unprotected oral sex (P=0.03), knowledge of asymptomatic syphilis (P=0.02), positive attitudes to mass treatment (P=0.02) and to the street nurses (P=0.01). Increased awareness was associated with increased condom use for vaginal sex overall (P=0.02) and for sex workers (P=0.03) and increased condom use for oral sex (P=0.05). There was no difference in syphilis incidence. Syphilis outbreak interventions that include education, support and referral can result in long-term positive behaviour changes.