Accessible summary• This article addresses the need for more appropriate sexual health assessment materials for use with women with a learning disability. • It gives a summary of other assessment measures that have been developed and why they are not suitable for women with a learning disability. • It describes the development of a new assessment measure that can be used to assess the sexual knowledge needs of women in secure settings, with a learning disability. • This research is important because it acknowledges that people with a learning disability have the same rights as others with regard to information about sexual health matters. As such people with a learning disability should be asked the questions about the sexual knowledge in an easy and straightforward way that allows them to discuss concerns. SummaryLittle research has been published on the need for sexual education for women in secure psychiatric facilities when patients present with a combination of learning disability, mental illness and personality disorder. The development of a brief, feasible, gender-specific sexual knowledge questionnaire is described, which has validity and reliability and which is comprehensible by 95% of adults. Results of a pilot study show significant sexual knowledge deficits in learning disability and mentally ill women along with risk and vulnerability factors. Ways of addressing these issues are discussed.