The frequency of thimerosal sensitization was determined in 5 groups of subjects: military recruits; healthy subjects; patients with asthma or rhinitis undergoing hyposensitizing therapy with thimerosal-preserved antigenic extracts; patients with allergic contact dermatitis; patients with allergic contact conjunctivitis. Our patients with allergic contact dermatitis have a higher incidence of positive patch tests to thimerosal than healthy subjects. The source of thimerosal sensitization in this group remains obscure, their history of exposure to this or to other mercury derivatives being comparable to that of the healthy population. Patients with allergic contact conjunctivitis and patients receiving immunotherapy for asthma or rhinitis present a significantly higher frequency of thimerosal sensitization than the other groups, indicating that ophthalmic solutions and thimerosal-preserved allergen extracts are an important source of thimerosal sensitization. The clinical relevance of thimerosal sensitization was definitively established only in patients with allergic contact conjunctivitis, even though patients receiving immunotherapy frequently presented local reactions at the site of allergen inoculation.