Twenty-nine children with typical Schönlein-Henoch purpura (SHP) were tested at the initial phase of the disease for respiratory function. Of the 29 patients, 28 had a decrease of lung transfer for carbon monoxide (TLCO) as measured by a steady-state method. Lung volumes and blood gas values were normal; slight radiologic signs of interstitial lung involvement were observed in 18 of 26 patients. There was a decrease in TLCO to 56.8% of normal values for height and gender and to 58.5% when normal values were volume-adjusted to functional residual capacity. In 19 of 25 patients, TLCO measurements were performed at 3-month intervals during follow-up. In all cases, normalization of TLCO values was observed only after complete clinical recovery from SHP. All children with persisting symptoms, even limited to microscopic hematuria or slight proteinuria, had low TLCO values. In one patient low TLCO during follow-up preceded a late relapse of SHP in the form of acute nephritic disease with characteristic IgA deposits on renal biopsy. We conclude that low TLCO in SHP is probably related to alteration of the alveolar-capillary membrane by circulating immune complexes. This noninvasive technique may be useful in diagnosis, and during the follow-up of the disease as an early indicator of reactivation.