OBJECTIVES: Insufficient preparation for children who are undergoing bone marrow aspiration can cause anxiety and negative outcomes. Nonpharmacological therapies have been proven to reduce fear in children who are undergoing painful procedures. We have therefore developed a mobile application to help reduce these patients’ anxiety by providing them with procedural information and coping skills. METHODS: This single-blinded, randomized controlled trial included 60 patients age 5 to 12 years old who were undergoing bone marrow aspiration procedures in Thailand that were conducted between May 2015 and May 2016. Sixty participants were randomly assigned to the intervention group (mobile application added to usual care) or the control group (usual care only). Preprocedural anxiety levels were evaluated by visual analog scales (child anxiety visual analog scale); this was repeated in the intervention group immediately after patients used the mobile application. On the day of the procedure, the patients’ cooperation levels were assessed by using the modified Yale Preoperative Anxiety Scale. The total amount of sedative drugs that were used was also recorded. The paired t test and the Wilcoxon signed rank test were used to analyze within-person change, whereas the t test and the Wilcoxon rank sum test were used for group comparisons. RESULTS: The child anxiety visual analog scale score of patients in the intervention group decreased significantly after they used the mobile application (P < .0012). The modified Yale Preoperative Anxiety Scale score of patients in the intervention group was significantly lower than that in the control group (P < .01). There was no difference in sedative use between the 2 groups. CONCLUSIONS: This mobile application possibly had effectiveness in routine use for reducing anxiety and increasing patients’ cooperation in bone marrow aspiration procedures.