Background The high prevalence of mental disorders among women in prison is recognised worldwide. In England and Wales, successive governments and independent reports have argued that the equivalent of community care in prisons is acceptable but that some mental health assessment units (MHAUs), staffed by professional clinicians, should remain. These have not been researched. Aims This paper aimed to explore patterns of use of a MHAU in a women's prison in England and to test the hypothesis that it was being used only, as intended – to hold women pending transfer to a health service hospital or in a bona fide crisis. Methods Anonymised data on all women transferred to one MHAU between 1 January 2008 and 31 August 2010 were obtained from the prison files and subjected to descriptive analysis. Results Less than a third of these women were transferred to an outside hospital; this group stayed longest in the unit. An overlapping group of 52% of the women was under a special assessment, care in custody and teamwork protocol because of suicide or serious self‐harm risk. Thus, 188 (68%) admissions fulfilled national protocol criteria for MHAU admissions. Two in five women admitted were released or returned to ordinary prison locations. Nevertheless, over 80% of the women were known to external mental health services, and 64 (30%) were so unwell on arrival in prison that they were transferred directly to the MHAU. Over a third of admissions were of women admitted more than once during the 32 months of study, and this was significantly more likely after release from prison directly to the community. Conclusions Our hypothesis was not sustained, and it seems unlikely that this prison MHAU is unique in being used outside its strict remit. A shift from studying the epidemiology of mental disorder in prisons to the epidemiology of mental health needs could benefit this vulnerable group and the wider community alike. Copyright © 2015 John Wiley & Sons, Ltd.