This study aimed to determine whether the Roper, Logan & Tierney model was an appropriate model for planning nursing care for clients who are mentally ill. Data were collected from two sources in one Health Board region in Ireland. A care plan audit was used to evaluate the extent to which the Roper, Logan & Tierney model was used to assess, plan and evaluate nursing care in nursing documentation. Qualitative interviews with nurses explored their experiences of using the model and their perceptions of the model's usefulness and appropriateness for planning care. Both data sets were complementary, the qualitative data often providing contextual information which helped put the findings into perspective. It was found that there was little evidence that the Roper, Logan and Tierney model guided care planning and that goals and nursing interventions were frequently not explicitly documented. Interviews with nurses indicated that they lacked educational preparation for using the model and found the model constraining and physically orientated. The appropriateness of the Roper, Logan and Tierney model for planning care for clients who are mentally ill is questioned. It is suggested that nurses need to be adequately prepared if they are to use a model appropriately. Consideration should be given when selecting a model as to its 'fit' with the needs of the client group and the ward team philosophy.