BACKGROUND: Bisexual- and lesbian-identified women have significantly worse mental health than heterosexual women. Less evidence exists about mental health differences between lesbian and bisexual women. METHODS: Self-completion survey with community-based, opportunistic sampling recruited 937 bisexual-identified and 4769 lesbian-identified women. Associations between sexual identity and mental health indicators were assessed by logistic regression, controlling for age, income, student status and employment. RESULTS: As a group, bisexual women were younger, poorer, and more likely to be trans-identified, minority ethnic identified and to use marijuana, compared with lesbians. Bisexuals were more likely than lesbians to report eating problems (adjusted odds ratio (AOR) = 1.64, P < 0.001), self-harm (AOR = 1.37, P = 0.001), depressed feelings (AOR = 1.26, P = 0.022) and anxiety (AOR = 1.20, P = 0.037). Fewer bisexual women attended lesbian or bisexual social events, were 'out', or had experienced any sexuality-related discrimination, compared with lesbians. CONCLUSION: More bisexual women reported poor mental health or psychological distress than did lesbians. Bisexual women may be more likely to experience social stress due to the 'double discrimination' of homophobia and biphobia. This stress, experienced mainly as internalized and felt stigma, could result in greater risk for poor mental health compared with lesbians. Addressing both biphobia and homophobia within UK society has important preventative mental health implications.