Objective: We sought to determine the prevalence of, and identify factors associated with, parent questions about physical activity for their child with a chronic cardiac, respiratory, or rheumatologic condition. Methods: This cross-sectional study of 56 children (32 [57%] female), 3 to 18 years of age, with chronic cardiac (n=21), respiratory (n=18), or rheumatologic (n=17) conditions involved a parent questionnaire about their child's physical activity participation and measurement of the child's activity with an omni-directional accelerometer for 1 week. Results: Parents of 20 (36%) children had at least one question about their child's physical activity participation, and the prevalence of questions did not vary by age (Wald chi square = 0.77, P=0.38), gender (Wald chi square = 0.11, P=0.74), or clinic (Wald chi square = 1.77, P=0.41). Parent questions were associated (P = 0.04) with lower levels of activity for boys (95% confidence interval [CI] for estimated marginal means: With questions: 197, 395; Without questions: 346, 500) and higher levels of activity for girls (95% CI for estimated marginal means: With questions: 268, 448; Without questions: 239, 369). A multivariable logistic regression model found that parents with questions had higher odds of having a child who was less well (odds ratio [OR]=19.9 for unwell, OR=5.6 for generally well with some symptoms versus well and asymptomatic) and had a history of cardiac arrhythmia (OR=7.6). Conclusions: Over one-third of parents reported having questions about physical activity for their child with a chronic medical condition, suggesting substantial uncertainty even among children reported as active. Presence of parent uncertainty is associated with parent reports of the child being unwell or a history of cardiac arrhythmia. By asking parents if they have questions about their child's