BACKGROUND: In 2008, the Mental Health Act (MHA) 2007 amendments to the MHA 1983 were implemented in England and Wales. The amendments were intended to remove perceived obstacles to the detention of high risk patients with personality disorders (PDs), sexual deviance and learning disabilities (LDs). The AMEND study aimed to test the hypothesis that the implementation of these changes would lead to an increase in numbers or proportions of patients with these conditions who would be assessed and detained under the MHA 2007. METHOD: A prospective, quantitative study of MHA assessments undertaken between July-October 2008-11 at three English sites. Data were collected from local forms used for MHA assessment documentation and patient electronic databases. RESULTS: The total number of assessments in each four month period of data collection varied: 1034 in 2008, 1042 in 2009, 1242 in 2010 and 1010 in 2011 (n = 4415). Of the assessments 65.6% resulted in detention in 2008, 71.3% in 2009, 64.7% in 2010 and 63.5% in 2011. There was no significant change in the odds ratio of detention when comparing the 2008 assessments against the combined 2009, 2010 and 2011 data (OR = 1.025, Fisher's exact Χ 2 p = 0.735). Only patients with LD and 'any other disorder or disability of the mind' were significantly more likely to be assessed under the MHA post implementation (Χ2 = 5.485, P = 0.018; Χ2 = 24.962, P > 0.001 respectively). There was no significant change post implementation in terms of the diagnostic category of detained patients. CONCLUSIONS: In the first three years post implementation, the 2007 Act did not facilitate the compulsory care of patients with PDs, sexual deviance and LDs.