Objectives Early syphilis in Multnomah County, Oregon, USA, increased 16-fold during 2007–2013. Cases predominantly occurred among men who have sex with men (MSM); 55% were HIV coinfected. We conducted a case–control study to evaluate the association between meeting sex partners online and early syphilis.

Methods Cases subjects (cases) were Multnomah County resident, English speaking, MSM, aged ≥18 years with laboratory-confirmed early syphilis reported 1 January to 31 December 2013. We recruited two MSM controls subjects (controls) per case, frequency matched by HIV status and age. Participants completed self-administered questionnaires. We performed multivariable logistic regression.

Results Seventy per cent (40/57) of cases and 42% (50/119) of controls met partners online (p<0.001). Cases more frequently met partners online (adjusted OR (aOR)=3.0; 95% CI 1.2 to 6.7), controlling for presumptive confounders. Cases reported more partners than controls (medians 5, 2; p<0.001). When including number of partners, aOR decreased to 1.4 (95% CI 0.5 to 3.9).

Conclusions Early syphilis was associated with meeting partners online. We believe this association may be related to number of sex partners acting as an intermediate variable between use of online resources to meet sex partners and early syphilis. Online meet-up sites might represent areas for public health interventions targeting at-risk individuals.