Among the more than 9.25 million people currently being held in penal institutions worldwide, mentally disordered inmates constitute a serious problem. Despite the standard doctrine in most countries that mentally ill offenders lacking criminal responsibility have to be referred to in forensic psychiatric facilities for specialised care, the prevalence of psychiatric morbidity among prisoners by far exceeds the rate of mental disorders in the general population. However, international research on this issue is scarce. The EU has recently funded a study on the overall concepts and capacities of mental health care in prisons from 25 European countries. Hindered by very bad health reporting standards, the study revealed deficient mental state screening and assessment procedures at prison entry as well as during the term or prior to release. This is an obstacle for implementing adequate primary, secondary or tertiary prevention programmes for most prevalent mental disorders in prisons. Additionally it increases the risk of relapsing and re-offending of released prisoners. None of the included countries provides regular national statistics on the frequency of mental disorders of prisoners or on the availability of psychiatric treatments. Together with a mostly unknown share of general psychiatry involvement into prison mental health care, the information gap currently prevents from identifying favourable concepts of prison mental health care in Europe. The paper presents the data collected during the study as well as a suggested set of standardized indicators whose implementation would enable to describe and evaluate this seriously neglected field.