We examined the 12-month prevalence of asthma and wheezing among U.S. children and compared the illness-related burden of children who wheezed with and without an asthma diagnosis. Data were obtained in a cross-sectional telephone survey that tested the performance of a health interview designed to identify children with chronic health conditions. Respondents were 712 primary caretakers of 1388 children under 18 years old in a national probability sample selected by random-digit dialing. Although 51 children identified with asthma and wheezing had more episodes, sleep disturbances, and attacks that limited speech, and received more medical treatment for wheezing than 69 children with wheezing alone, the "undiagnosed" children appeared to be only somewhat less affected by their wheezing. Repeat episodes and burden experienced by children with wheezing alone suggest that the asthma prevalence may be underestimated if based only on the diagnostic label.