OBJECTIVES: Outpatient care is critical in the management of chronic diseases, including sickle cell disease (SCD). Risk factors for poor adherence with clinic appointments in SCD are poorly defined. This exploratory study evaluated associations between modifying variables from the Health Belief Model and missed appointments. METHODS: We surveyed adults with SCD (n = 211) and caregivers of children with SCD (n = 331) between October 2014 and March 2016 in six centres across the U.S. The survey tool utilized the framework of the Health Belief Model, and included: social determinants, psychosocial variables, social support, health literacy and spirituality. RESULTS: A majority of adults (87%) and caregivers of children (65%) reported they missed a clinic appointment. Children (as reported by caregivers) were less likely to miss appointments than adults (OR:0.22; 95% CI:(0.13,0.39)). In adults, financial insecurity (OR:4.49; 95% CI:(1.20, 20.7)), health literacy (OR:4.64; 95% CI:(1.33, 16.15)), and age (OR:0.95; 95% CI:(0.91,0.99)) were significantly associated with missed appointments. In all participants, lower spirituality was associated with missed appointments (OR:1.83; 95%CI:(1.13, 2.94)). The most common reason for missing an appointment was forgetfulness (adults: 31%, children: 26%). A majority thought reminders would help (adults: 83%, children: 71%) using phone calls (adults: 62%, children: 61%) or text messages (adults: 56%, children: 51%). CONCLUSIONS: Our findings demonstrate that modifying components of the Health Belief Model, including age, financial security, health literacy, spirituality, and lacking cues to action like reminders, are important in missed appointments and addressing these factors could improve appointment-keeping for adults and children with SCD.