OBJECTIVES: To determine which prognostic information sources parents find informative and which are associated with better parental understanding of prognosis. METHODS: Prospective, questionnaire-based cohort study of parents and physicians of children with cancer at 2 academic pediatric hospitals. We asked parents how they learned about prognoses and evaluated relationships between information sources and prognostic understanding, defined as accuracy versus optimism. We excluded parents with pessimistic estimates and whose children had such good prognoses that optimism relative to the physician was impossible. Analytic cohort of 256 parent-physician pairs. RESULTS: Most parents considered explicit sources (conversations with oncologists at diagnosis, day-to-day conversations with oncologists, and conversations with nurses) "very" or "extremely" informative (73%-85%). Implicit sources (parent’s sense of how child was doing or how oncologist seemed to feel child was doing) were similarly informative (84%-87%). Twenty-seven percent (70/253) of parents reported prognostic estimates matching physicians’ estimates. Parents who valued implicit information had lower prognostic accuracy (odds ratio [OR] 0.50; 95% confidence interval 0.29-0.88), especially those who relied on a "general sense of how my child’s oncologist seems to feel my child is doing" (OR 0.47; 0.22-0.99). Parents were more likely to use implicit sources if they reported receiving high-quality prognostic information (OR 3.02; 1.41-6.43), trusted the physician (OR 2.01; 1.01-3.98), and reported high-quality physician communication (OR 1.81; 1.00-3.27). CONCLUSIONS: Reliance on implicit sources was associated with overly-optimistic prognostic estimates. Parents who endorsed strong, trusting relationships with physicians were not protected against misinformation.