The possible causative relationship between exposure to house dust mite allergen (HDMA) and symptoms of vernal keratoconjunctivitis (VKC) was investigated. VKC patients were evaluated for an average of 18 months using patient reports of symptoms, physicians’ observations and assays of sera and tears for total and specific IgE to 10 inhalant allergens common in Israel, including HDMA. Tear and serum samples from a group of 8 patients reacted only with HDMA, and were compared with the tear and serum samples of a group of 11 patients not reacting to any of the above allergens. House dust samples from the homes of two VKC patients in the HDMA-reactive group were examined monthly for a year. While mites were detected throughout the year, they were approximately 3 times more abundant from June through September than at other times. 96.6% of the mites collected belonged to the family Pyroglyphidae; 91.0% were Dermatophagoides pteronyssinus and 5.5% Dermatophagoides farinae. Both the severity of VKC symptoms in HDMA-reactive patients and mite population levels peaked simultaneously in the summer. This relationship was not seen among the non-HDMA-reactive patients. Our results suggest that exposure to HDMA plays an important role in the aggravation of VKC symptoms in HDMA-reactive patients.