Psychosocial outcomes are important in the perspective of boys with haemophilia. However, health-related quality of life (HRQoL) is based on self-report, and assumes adequate literacy. Yet, literacy is rarely assessed prior to data collection. This study sought to identify criteria that might indicate the level of literacy of children being recruited for clinical trials and to develop a simple method to prescreen those whose literacy was uncertain. We developed a brief screening tool in the form of two stories, at a grade 3 reading level, followed by comprehension questions. We applied the screening test to a sample of haemophilic boys between the ages of 7 and 13 years to assess their literacy. The data were analysed to determine the best criteria to use in identifying the ability to independently self-report for HRQoL studies. Twenty-four Brazilian boys (7.9–12.8) completed the testing. The results showed that 17 (70.8%) were literate (were able to both read and comprehend), and could complete a questionnaire without assistance. All boys over 11.0 years of age were sufficiently literate. Grade level was not found to be a helpful criterion. We recommend that all children under the age of 11.0 years be prescreened before providing self-reported HRQoL data. Those with limited literacy should be provided assistance to ensure comprehension of the questions. This is important to ensure high-quality data on HRQoL for future clinical trials.