The inhabitants of a fruit growing area often report spontaneously of sensitization to the red spider mite (RSM) (Panonychus ulmi KOCH). These are for the most part sensitizations with low clinical symptoms (rhinitis, conjunctivitis and erythema). Severe clinical developments with bronchial asthma have been observed. We investigated six patients working in a fruit growing area sensitized by RSM. The sensitizations corresponded to a Type I allergy. Skin tests and provocation tests (nasal as well as bronchial) with RSM showed immediate reactions and RAST positive results were obtained using RSM allergen disks. RAST measurements of sera from nine house-dust mite Dermatophagoides pteronyssinus allergic patients using RSM allergen disks showed RAST-class > 1 for eight patients. RAST inhibition and immunoprint suggest a possible cross-reaction between RSM and D. pteronyssinus.