PURPOSE: Physical inactivity is a global health concern, further magnified by gender disparity, with women being less active than men. Gyms may be optimal environments to engage in physical activity, however, perceived gendering describe a major barrier for women to become and stay members. Therefore, our aim was to compare attendance behaviour over the first 12 months and membership length (ML) of female gym members in mixed-gender gyms to a female-only gym, to identify factors explaining both behaviours. METHODS: Data from one female-only (n=1,881) and eight mixed-gender gyms (n=21,133 women) across England were analysed, including demographics, membership characteristics, and each member’s visits at the gym between 2008-2019. Data were analysed via t-tests and mixed-effect regression analyses using SPSS and R, with significance set at p=0.05. RESULTS: Women in female-only gyms showed a higher average ML (19.59±21.91 v. 14.30±16.03 months, p<0.001) and higher average attendance frequency (4.33±3.29 v. 2.70±3.15 visits/month, p<0.001), compared to women in mixed-gender gyms. In female-only gyms, monthly visits and ML were inversely related (r=-0.441, p<0.001) and the calendar month in which the membership commenced (r=-0.065, p<0.001), while a higher age increased attendance (r=0.031, p<0.001) (R2=0.147). An increase in ML was associated with a higher number of membership freezes (r=3.660, p<0.001), and higher attendance frequency during the 11th (r=0.619, p<0.001) and 12th (r=0.598, p<0.001) month of membership (R2=0.191). In mixed-gender gyms, attendance also decreased with ML and increased with age (r=-0.217, p<0.001 and r=0.011, p<0.001). A multitude of factors were associated with ML in women in mixed gender gyms, including age (r=-0.006, p<0.001), attendance frequency during the first month (r=-0.062, p<0.05), month 11 (r=0.241, p<0.001) and month 12 (r=0.400, p<0.001) (R2=0.103). CONCLUSIONS: Women in female-only gyms show a higher attendance frequency during the first 12 months and a longer ML than women in mixed-gender gyms. Demographic and membership-related factors only explained a small proportion of the heterogeneity in both outcomes. However, observed R2’s were higher in female-only gyms, indicating that other factors have a stronger impact on women in mixed-gender gyms.