AIM: The aim of this survey was to investigate the attitudes, knowledge and reported practice (capabilities) of mental health workers concerning human immunodeficiency virus (HIV) and other sexually transmitted diseases in people with serious mental illness. BACKGROUND: People with serious mental illness are at increased risk of HIV and other sexually transmitted infections. Mental health workers have a key role to play in promoting sexual health in this population, but it is unclear how they perceive their role in this work and whether they have the capabilities to deliver sexual health promotion. DESIGN: Cross sectional survey. METHODS: A questionnaire was devised and distributed to 650 mental health workers working in a London (UK) NHS mental health service. RESULTS: A response rate of 44% was achieved. Overall, workers reported positive attitudes to sexual health promotion and were knowledgeable about risk behaviours and risk factors for HIV infection. Adherence to glove wearing was good. However, participants' knowledge about HIV/AIDS in people with schizophrenia was poor and most reported they were not engaged in sexual health promotion activities with people with serious mental illness. Glove wearing was predicted by those who had drug and alcohol training and clinical experience and knowledge of risk factors was predicted by previous health promotion training. No other demographic factors predicted any of the other subscales. CONCLUSION: Mental health workers require training to provide skills for health promotion regarding sexual health and HIV in people with serious mental health problems. In addition, there needs to be more research on risk behaviours. RELEVANCE TO CLINICAL PRACTICE: The development of effective interventions to reduce this behaviour.