This study was conducted to examine a patient's age and condition at the time of diagnosis as one potential factor contributing to the "gender gap" in cystic fibrosis. The study population consisted of 11,275 US patients diagnosed during 1986-1998 and reported to the Cystic Fibrosis Foundation Registry in the same or the following calendar year. Parallel analyses were performed for Wisconsin patients identified prospectively during 1985-1994 to obtain more detailed information on their condition at diagnosis. Analyses of the registry data showed that females identified because of symptoms other than meconium ileus were diagnosed at significantly older ages (median, 12.7 months) than were males (median, 8.7 months) (p < 0.001). The delay in diagnosis for females was most evident among patients presenting with respiratory symptoms only (median, 40.7 vs. 22.3 months; p < 0.001). Analyses of Wisconsin patients demonstrated no significant gender differences in cough and wheezing experiences or in chest radiographic severity scores between males and females during their first 10 years of life, although a disproportionately high number of males were referred for diagnostic sweat testing. A delay in diagnosis of females with cystic fibrosis was discovered, suggesting either differential recognition of respiratory symptoms or a gender bias.