Esparto grass (Stipa tenacissima), which is commonly found in the Mediterranean area, has a wide variety of uses. Five plaster workers from the same family developed cough, dyspnea, malaise, and fever after exposure to the esparto fiber used in their work for the previous few years. They showed a significant decrease in symptoms when away from work. Precipitating antibodies against an esparto extract were found in the sera of all patients. Specific IgG antibodies against Aspergillus fumigatus were detected. A. fumigatus was identified after microbiologic evaluation of esparto fiber samples. The dust derived from fungi-contaminated esparto fibers can cause hypersensitivity pneumonitis in exposed subjects. The causative antigen is A. fumigatus. When esparto fibers were strongly contaminated by fungi, all the workers developed a clinical picture compatible with hypersensitivity pneumonitis. The coincidental finding of an occupational and a familiar condition is unusual.