Background: Studies have linked the behaviour problems of children with fragile X syndrome (FXS) to maternal well-being, but less is known about how behaviour problems relate to important family factors such as marital satisfaction and family cohesion. Methods: Married mothers of 115 adolescents and adults with FXS completed questionnaires and interviews, and maternal CGG repeat length was obtained by medical/laboratory records or by blood analysis. Results: Indirect effects were present between behaviour problems and family variables in that behaviour problems were positively related to maternal internalising symptoms which were, in turn, negatively associated with both family cohesion and marital satisfaction. Direct associations between behaviour problems and family relationship variables were not significant. Conclusions: Findings suggest the importance of intervening with behaviour problems in individuals with FXS and identify maternal mental health as a potentially powerful conduit for the effects of child behaviour on relationships within these families. Implications for targeted interventions are discussed. (Contains 2 tables and 1 figure.)