Hypernitrosopnea, a robust marker for childhood asthma, is measured reproducibly in mixed vital capacity (VC) expirates. Recent guidelines for measurement of expired nitric oxide (NO) in adults have favored use of an online (OL), flow-dependent technique. We compared VC and OL NO measurements in 14 asthmatic and 11 control children 5 through 18 yr of age. After spirometry, subjects breathed both into an open-ended reservoir (20 cm H2O resistance) and into a tedlar bag (VC maneuver). End-expiratory pressure > 5 cm H2O was continuously maintained during VC measurements, and the velum remained shut. Eight additional children (24% of total number of subjects) were unable reproducibly to perform the OL measurement at constant flow (six asthmatics; two control children). For subjects able to perform the OL technique, OL and VC NO measurements were linearly related (r2 = 0.88). In children, VC NO assays are reproducible, sensitive in identifying asthma, and portable. Additionally, we have shown that (1) not all children are able to perform OL measurements, and (2) VC measurements vary linearly with OL measurements. These findings suggest that there may not be compelling reason to favor OL over VC measurements for hypernitrosopnea in children with asthma.