Objective: This study explored the utility of the health belief model (HBM) in explaining medication adherence in subjects with severe and disabling mental disorders. Method: Six well‐established measuring instruments, with confirmed reliability and validity, were used to assess each component of the HBM and medication adherence in 39 hospital‐treated subjects with affective disorders (n=27) or schizophrenia (n=12). Results: Highly adherent and partially adherent subjects differed significantly in their perception of illness severity, their beliefs about themselves and their control over the disorder, and their concerns about further hospitalization. Two components of the HBM (perceived severity of illness and perceived benefits of treatment) explained 43% of the variance in adherence behaviour. Conclusion: Although the study has a number of methodological limitations, the results suggest that clinical assessment of components of the HBM may improve the detection of patients at risk of medication non‐adherence.