Background: Fire-setting and self-harm behaviours among women in high security special hospitals may be understood using Shye's Action System Theory (AST) in which four functional modes are recognized: 'adaptive', 'expressive', 'integrative', and 'conservative'. Aims: To test for relationships between different forms of fire-setting and self-harm behaviours and AST modes among women in special hospital, and for consistency within modes across the two behaviours. Method: Clinical case files evidencing both fire-setting and self-harm behaviours (n = 50) were analysed for content, focusing on incident characteristics. A total of 29 fire-setting and 22 self-harm variables were analysed using Smallest Space Analysis (SSA). Chi-square and Spearman's rho (ρ) analyses were used to determine functional consistency across behavioural modes. Results: Most women showed one predominant AST mode in fire-setting (n = 39) and self-harm (n = 35). Significant positive correlations were found between integrative and adaptive modes of functioning. The lack of correlation between conservative and expressive modes reflects the differing behaviours used in each activity. Despite this, significant cross-tabulations revealed that each woman had parallel fire-setting and self-harm styles. Discussion: Findings suggest that, for some women, setting fires and self harm fulfil a similar underlying function. Support is given to AST as a way of furthering understanding of damaging behaviours, whether self- or other-inflicted.