Soluble immune complexes were detected by a solid phase Clq binding assay in forty-two out of 106 well studied asthmatic patients (39.6%) and in eleven out of 145 age-matched controls (8%; P < 0.01). Clinical significance of immune complexes has been evaluated by comparing the following parameters in patients with and without such complexes: age, sex, duration of disease, IgE-mediated allergy (RAST, skin test), precipitins (Aspergillus, Candida albicans, Thermoactinomycetes), corticodependency, lung function tests, associated symptoms (hivernal bronchitis, urticaria, eczema, …), desenstization treatment, serum concentration in immunoglobulin in G, A, M and E. Immune complexes were found more frequently in female than in male patients. The prevalence of immune complexes was higher in patients treated by hyposensitization therapy and in cases with precipitating antibodies against thermophilic actinomycetes and Aspergillus.