To prevent summer-type hypersensitivity pneumonitis, the effect of eliminating Trichosporon cutaneum from patients' homes was investigated. To determine environmental factors associated with enhanced indoor mold growth, we compared 11 patients' homes in which T. cutaneum was isolated to 32 control homes by inspection and indoor mold survey. Statistically significantly higher isolates of T. cutaneum, total yeasts, and total fungi were associated with unsanitary rooms, and especially with damp places in the wood or tatami mats in patients' homes. Cleaning, disinfecting, and removing the colonizing locations of T. cutaneum were performed. No recurrence was observed in 6 well-cleaned homes, but recurrence was observed in all patients who resided in homes that were not cleaned or in homes where cleaning was not adequate, and where T. cutaneum was not eliminated. The efficacy of therapy by elimination of T. cutaneum was proven.