Objective: This study was carried out to evaluate whether patients with cystic fibrosis (CF) who are socioeconomically disadvantaged, have a different outcome than higher-income patients. Medicaid insurance coverage was used as a proxy of low family income. Study design: A cross-sectional analysis was performed on a group of patients younger than 21 years of age seen at the University of North Carolina CF Center in 1994. Results: The difference in median age at diagnosis for Medicaid (132 days) and non-Medicaid (177 days) patients was not statistically significant (p = 0.17). Medicaid patients' mean maximum percent predicted forced expiratory volume in 1 second for the year was 11.6% less than that of the non-Medicaid group (p = 0.01); this difference was constant across age. Medicaid patients averaged 0.8 more admissions for the year (p < 0.01) and 8.8 more days (p < 0.01) in the hospital but received the same amount of outpatient intravenous therapy. They also had 1.20 more outpatient visits to the center during the year (p = 0.02). Conclusions: Medicaid patients with CF have worse lung function and require more treatment for pulmonary exacerbations than their more advantaged counterparts. This difference appears to begin early in life, does not increase with age, and is not explained by inadequate access to outpatient specialty care or delayed diagnosis. Other explanations such as inadequate access to primary care, poor adherence to prescribed regimens, or greater exposure to pollutants (e.g., environmental tobacco smoke) are speculative. (J Pediatr 1998;132:260-4)