Objectives: To assess the frequency and characteristics of missed opportunities for influenza immunization in children with chronic medical conditions and,among unimmunized children in that group, to explore parent-reported reasons for not vaccinating their child. Design: Prospective cohort study. Data were obtained from billing and immunization registry databases and telephone interviews of parents. Setting: Four pediatric practices in metropolitan Denver, Colo, during the 2002-2003 influenza season. Participants: Children aged 6 to 72 months with 1 or more chronic medical conditions. Main Outcome Measure: A missed opportunity for influenza immunization, defined as having a billed encounter, being eligible for immunization, and not receiving vaccine. Subjects with asthma were analyzed separately from those with other conditions. Results: We identified 926 children with chronic conditions: 820 (89% )-with asthma only and 106 (11% ) with other conditions. Missed opportunities occurred at 78% of all vaccine-eligible visits for children with asthma and 74% of all visits for children with other conditions. For children with asthma, 92% of vaccine-eligible visits in December and January resulted in a missed opportunity vs 69% in October and November; for children with other conditions, corresponding frequencies were 86% vs 68% , respectively. For children with asthma, 86% of non-well-child visits resulted in a missed opportunity, compared with 62% of well-child visits; similar frequencies were seen among children with other chronic conditions. Parents of unimmunized children reported lack of a physician recommendation and low perceived susceptibility to influenza as the primary reasons for not immunizing their child. Conclusion: Missed opportunities contribute significantly to low influenza immunization rates among children with chronicmedical conditions. Objectives: To assess the frequency and characteristics of missed opportunities for influenza immunization in children with chronic medical conditions and, among among unimmunized children in that group, to explore parent-reporting reasons for not vaccinating their child. Design: Prospective cohort study. from billing and immunization registry databases and telephone interviews of parents. Setting: Four pediatric practices in metropolitan Denver, Colo, during the 2002-2003 influenza season. Participants: Children aged 6 to 72 months with 1 or more chronic medical conditions. Main Outcome Measure : A missed opportunity for influenza immunization, defined as having a billed encounter, being eligible for immunization, and not receiving vaccinations. Results: We identified 926 children with chronic conditions: 820 (89 %) -with asthma only and 106 (11%) with other conditions. Missed opportunities occurred a t 78% of all vaccine-eligible visits for children with asthma and 74% of all visits for children with other conditions. For children with asthma, 92% of vaccine-eligible visits in December and January resulted in an missed opportunity vs 69% in October and November; for children with other conditions, correspondingrations were 86% vs 68%, respectively. For children with asthma, 86% of non-well-child visits resulted in a missed opportunity, compared with 62% of well-child visits ; similar frequencies were seen among children with other chronic conditions. Parents of unimmunized children reported lack of a physician recommendation and low perceived susceptibility to influenza as the primary reasons for not immunizing their child. Conclusion: Missed opportunities contributed significantly to low influenza immunization rates among children with chronicmedical conditions.