Chironomid larvae (red midge larvae) are often used by aquarists as fish food. Their hemoglobins can cause IgE-mediated allergic diseases in exposed and unexposed people. The aim of this study was to find out the prevalence of positive skin tests to chironomids in patients suffering from rhinitis and/or bronchial asthma in Ciudad Real (Spain). A total of 465 patients were submitted to skin prick tests with chironomids in addition to common inhalant allergens. The patients with positive skin prick test (wheal > or = 3 mm) to these larvae answered a questionnaire. Skin prick tests with Acarus siro, shrimp, cockroach and mosquito (Culex pipiens) were carried out. Serum levels of total IgE and anti-Chironommus thummi, anti-Dermatophagoides pteronyssinus, anti-shrimp, and anti-mosquito (Aedes communis) IgE were determined. Conjunctival or nasal provocation tests were carried out with chironomids. Of the 465 patients skin tested, 19 showed a positive skin prick test with chironomids, corresponding to 4.1% of all patients and 6% of the atopic patients. None were monosensitized. Significant correlations were found between skin prick test results with chironomids and mites (p < 0.005). Of the 19 patients, 15 had positive skin prick test with the common mosquito C. pipiens. Seven patients showed elevated anti-D. pteronyssinus IgE, six elevated anti-shrimp IgE, and 10 showed elevated anti-A. communis IgE. Provocation tests with chironomids were positive in 14 patients (four nasal and 10 conjunctival tests). Conjunctival provocation tests were carried out in 16 controls and were positive in three; all three showed positive skin prick test with chironomids. One patient had occupational allergy from the larvae (aquarist). After mosquito bites, five patients showed immediate wheal reactions and one patient suffered an anaphylactic reaction after several mosquito bites. Only two patients remembered having been in contact with chironomids as fish food. We found hypersensitivity to these larvae in patients without apparent contact to them. These patients could have become sensitized in various ways, including: 1) inhaling particles of chironomids or others that are cross-reactive with them; 2) exposure to products used as fish food containing chironomids; and, 3) through cross-reactivity with other allergens such as mites, shrimp or mosquitoes.