Background Being a current psychiatric in-patient is one of the strongest statistical risk factors for suicide. It is usually assumed that this strong association is not causal but is a result of the combination of the selection of high-risk patients for admission and the imperfect protection from suicide afforded by psychiatric wards. Logically, a third factor, which is causal, might play a role in the association. It has recently been suggested that adverse experiences in psychiatric units such as trauma, stigma and loss of social role might precipitate some in-patient suicides. Aims To consider whether there is a causal association between psychiatric hospitalisation and suicide. Method We used the framework of Austin Bradford Hill's criteria for assessing causality in epidemiology to consider the possibility that psychiatric hospitalisation might causally contribute to the extent and variation in in-patient suicide rates. Results The association between psychiatric hospitalisation and suicide clearly meets five of the nine Hill's criteria (strength of association, consistency, plausibility, coherence and analogy) and partially meets three of the remaining four criteria (gradient of exposure, temporality and experimental evidence). Conclusions Admission to hospital itself might play a causal role in a proportion of in-patient suicides. The safety of being in hospital with respect to suicide could be examined with a large-scale randomised controlled trial (RCT). In the absence of an RCT, the possibility of a causal role provides further impetus to calls to make care in the community more available and psychiatric hospitals more acceptable to patients.