Rates of mental illness and self-harm are very high among women prisoners. Questionnaires assessed prisoners' knowledge of and attitudes towards mental health problems, and relevant behavioural intentions before and after the intervention, to evaluate the effectiveness of a comedy show in a women's prison to reduce mental health stigma and improve coping and help-seeking for mental health problems. The intervention appeared to have been successful in improving some aspects of prisoners' knowledge about the effectiveness of psychotherapy (Z = - 2.304, p = 0.021) and likelihood of recovery from mental health problems (Z = - 2.699, p = 0.007). There were significant post-intervention increases in the proportion who stated they would discuss or disclose mental health problems with all but one of the sources of help in the questionnaire, which was consistent with the increases in the number of prisoners who rated themselves as likely to start using different sources of help or prison activities. There was no improvement in intentions to associate with people with a mental health problem. The intervention appeared effective in improving factors that might increase help-seeking and improve coping, but not those that would change behaviour towards others with a mental health problem.