This paper explores the complexities of medical social work undertaken with step families caring for a child suffering from cancer. Although there is a growing literature dealing with the psycho-social effects of childhood cancer, there is an unexamined assumption that children are cared for by their birth parents within a traditional nuclear family. It is argued that the paediatric oncology social worker is challenged increasingly to develop understandings and modes of intervention which are appropriate and sympathetic to the particular issues lacing step families. Using a systemic and chronological framework from referral and diagnosis through to bereavement, the authors identify challenges for the step family, corresponding implications for social work practice, and questions for future research. [ABSTRACT FROM AUTHOR]