To determine the prevalence and severity of childhood asthma in New Zealand we studied 815 children from a birth cohort by questionnaire, clinical examination and pulmonary physiological measurements at age 9 years. More than 19% of the sample had experienced wheezing in the previous year, and 11% had wheezed in the month before assessment. In all, 220 of 815 children (27.1%) had had wheezing episodes by age 9; in 34 (4.2%) episodes had been of sufficient frequency to warrant regular anti-asthma treatment. Only 32% of all wheezing children were reported by their parents to suffer from asthma, and, in groups matched for frequency of symptoms, treatment given for wheezing was influenced strongly by whether or not the label of 'asthma' had been given. The detailed history provided most information useful in diagnosing asthma; clinical examination, peak flow records, spirometry and bronchial provocation testing provided only limited additional information. A wide spectrum of frequency and severity of recurrent wheezing disorders was evident in New Zealand children. Throughout all degrees of severity, prevalence rates appeared higher than those reported in studies from the United Kingdom.