Attrition in longitudinal studies of survivors of childhood cancer reduces these studies' statistical power, introduces bias and threatens internal and external validity. This study investigated the variables associated with dropout of survivors of acute lymphoblastic leukemia in a trial investigating the effect of vitamin D and calcium supplementation and nutritional counseling on bone mineral density (BMD). Twenty-five participants withdrew from the study. Common reasons given for withdrawing were intolerance of the study drug, family hardship and schedule conflicts. Few statistically and clinically significant differences identified participants who completed the study. Nurses need to be aware of the reasons that participants withdraw from clinical trials, as they are in a strategic position to encourage patients to participate in health promotion studies.