Non-isotonic aerosols are being used more commonly to perform bronchial provocation tests. In contrast to histamine and methacholine challenge tests they appear to have higher specificity and a higher predictive value. The aim of the present study was to assess the reproducibility of the response to challenge with 4.5% hypertonic saline administered via an ultrasonic nebulizer in children with mild to severe asthma. Seventeen children with asthma aged 10 to 14 years completed two or three challenge tests at the same time of day within a 10 day period. Of these 17 children 9 had mild, 4 moderate, and 4 severe asthma. Children inhaled 4.5% hypertonic NaCl from an ultrasonic nebulizer with an output of 1.9 to 2.5 mL/min (Timeter) using the protocol developed by Anderson with modifications. A fall in forced expiratory volume in 1 second (FEV1) from baseline of 15% or more was considered a positive response and PD15 was calculated. In 16/17 subjects a greater than 15% fall in FEV1 occurred consistently on all study days. One subject with moderate asthma had a less than 15% fall in FEV1 on both study days. The coefficient of repeatability for PD15 was 1.8. This equals 0.85 of a doubling dose difference between the two or three measurements of PD15. The cumulative time of aerosol inhalation causing a fall in FEV1 > or = 15%) (PT15) showed a coefficient of repeatability of 1.59, similar to 0.67 doubling dose difference. The PD15 and PT15 were highly significantly correlated. The 4.5% NaCl challenge test yields good reproducibility in children with mild to severe asthma under laboratory conditions.