The efficacy of the acaricide benzyl benzoate as an additive to a chemically and technically defined cleaning substance (Acarosan®) was tested in a multicentric, prospective, randomized, controlled study on 118 outpatients with bronchial asthma due to house-dust-mite allergy. Subjective reports from patients and doctors revealed an improvement in clinical complaints in more than 50%. with only small differences between the verum and the placebo group. Objective parameters such as titrated skin tests, RAST. and bronchial challenge tests with histamine and Dermatophagoides pteronyssinus (D. pt.) did not reveal any significant changes either during the year of testing or between the two groups. A clinical improvement as observed in either group could not be assessed by objective parameters. Additional questions as to the merits of the possible prophylactic use of benzyl benzoate over more than 1 year remain unanswered.