OBJECTIVE Severe and chronic mental illness (SMI) is associated with complex care needs. Mental health care planning requires exact estimates of the number of SMI patients. METHODS Systematic search for conceptual and epidemiological work on SMI prevalence (age group 18 - 65 years) in Medline, Embase and PsycInfo. RESULTS To date there is no international consensus on the definition of severe mental illness, however, there are a number of international studies allowing an estimate for Germany. The number of those affected with a severe mental illness based on the present findings is estimated at 1% to 2% of adults between 18 and 65 years. This means, that in Germany, about 500,000 to 1 million people are in need for complex mental health care interventions. CONCLUSIONS The numbers estimated may serve as a first orientation in the field. Further research on SMI prevalence is urgently needed in order to inform a more precise mental health care planning process for people with SMI in Germany.