Background: Haemophilia (hemophilia) is a chronic and inherited genetic disorder that can cause physical and mental problems, as well as harm the individual and social life of families. The present paper aimed to survey the role of perceived stress in families of patients with hemophilia and its role in the prediction of cognitive failures and mental health in patients with hemophilia. Methods: The present paper was a descriptive -correlational study. The population of the study comprised 370 patients with hemophilia and one of their companions in 2017. Using convenience sampling, 188 patients were chosen by the use of Morgan's formula. Data were gathered using cognitive failures, mental health, and perceived stress questionnaires. The data were analyzed in SPSS -16 using Pearson correlation and Stepwise Regression. Results:The results showed a significant positive correlation between cognitive failure of patients with hemophilia and negative perception of stress in families of patients with hemophilia (r = 0.195, P = 0.05). There was also a significant negative correlation between the mental health of patients with hemophilia and negative perception of stress in the families (r = 1.060, P < 0.05). Regarding the results of the regression analysis, negative perception of stress predicted 3.8% of changes in cognitive failure and 2.6% of changes in the mental health of patients with hemophilia. Conclusions:The findings showed that negative perception of stress in patients with hemophilia and their families could predict cognitive failure and mental health of these patients. Therefore, it is necessary to provide more access to strong social support networks and provide parents and patients with essential information about hemophilia, in addition to current hemophilia treatments.