The chemical composition of biologically active wood dusts was determined. Samples of dust obtained from five softwoods and 21 hardwoods were chosen for analysis because they were thought to be responsible for causing bronchial asthma and other respiratory problems in Italian woodworkers. Three softwoods and 10 hardwoods were native to Italy. The others were tropical or exotic woods. After extraction with 95% ethanol or 1% hydrochloric-acid the dusts were analyzed for polyphenols, terpenoids, cardenolids, alkaloids, anthranoids, and coumarins by thin layer chromatography. Eighty six patients being treated for bronchial asthma and other respiratory problems were prick tested with the dusts. Among the exotic woods, moderate amounts of polyphenols, cardenolids, and terpenoids were found. Alkaloids were found in Asian rosewood, Mansonian walnut, and teak. Anthranoids were found only in Mansonian walnut. Among the native woods, polyphenols and terpenoids were consistently found. Significant amounts of cardenolids, alkaloids, and anthranoids were not found. Approximately 45.3% of the patients had bronchial asthma. Chronic rhinopharyngitis occurred in 58.2% of the subjects and radiological alterations of the paranasal sinuses in 46.7%. An obstructive ventilatory defect was found in 50% of the patients and a mixed ventilatory defect in 15%. Only four subjects demonstrated a positive response to skin prick testing with wood dust. Two reacted to Tanganyika Aniegre and two reacted to cherry. One subject reacted to a mixture of Aniegre and Graminaceous pollen. One reacted positively to a mixture of pine, oak, and some mycophytes. Six subjects gave positive responses to bronchial challenge with the wood dusts. The wood dusts that provoked a positive reaction were Aniegre and Mansonian walnut. Three subjects reacted to Aniegre in both tests. Two of these demonstrated specific immunoglobulin-E antibodies toward Aniegre in additional testing.