Objectives. Attachment styles of forensic service users can be unclassifiable, agreement between staff regarding service user attachment styles may be poor. Instability of attachment behaviour may be associated with untoward incidents in a medium secure unit. Methods. In a two-stage study, firstly Fleiss Kappa determined agreement levels in attachment style ratings of 43 service users by 60 nursing staff in a medium secure clinic. In the second stage, nursing observations of attachment and untoward behaviour in 10 female service users were analysed over an average of 24 days using Generalised Estimated Equation modelling. Results. Staff ratings of attachment styles were inconsistent, service users were most frequently categorised in all 4 attachment style categories. Within-day variability in attachment behaviour in 10 female service users was common, increased untoward incidents were associated with the highest possible level of attachment behaviour variability (which included secure attachment behaviours). Conclusions. Fluid, exhaustive changes in attachment behaviour may be more useful than attachment styles for predicting challenging behaviour in secure care. Inclusion of securely attached behaviour in the pattern associated with incidents may be especially problematic for predicting proximal risk. Practitioner points