Background: Allergic Conjunctivitis (AC) has a high incidence in the general population and sometimes it is difficult to make a correct diagnosis, distinguish among the different subtypes of AC, and therefore, to indicate the suitable therapy. Objective: To determine the best way to carry out an appropriate diagnosis of AC. Methods: Thirty-one patients with clinical manifestations of AC and eleven controls were studied by measuring allergic and immunologic parameters. Only those patients confirmed as having AC were treated with ketotifen fumarate and further evaluated. Results: According to allergic and immunological parameters patients were divided rate two groups. Group I patients had positive prick test toward at least one allergen, 60% exhibited high levels of tear-IgE, and only 36% conjunctival eosinophils. By contrast, patients from Group II had negative prick tests and laboratory findings similar to the control group. In Group I there was a good correlation between levels of tear-IgE and eosinophils (r = 0.55; p = 0.009); key symptoms and signs and prick test (r = 0.52; p = 0.015), and prick test and eosinophils (r = 0.50 p = 0.022). The cardinal signs and symptoms scores dropped significantly in Group I as a consequence of the treatment (p < 0.0001). Conclusion: In order to have a reliable AC diagnosis, allergen-skin prick test. IgE in tears, and conjunctival eosinophils must be studied. Serum IgE is less important.Fil: Martín, A. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas; ArgentinaFil: Gómez Demel, E. Universidad de Buenos Aires. Hospital Privado; ArgentinaFil: Gagliardi, J. Universidad de Buenos Aires. Hospital Privado; ArgentinaFil: Berra, Alejandro. Universidad de Buenos Aires. Facultad de Medicina; ArgentinaFil: Mariani, A.L. Universidad Nacional de Misiones; ArgentinaFil: Baena Cagnani, C.E. Hospital Infantil Municipal de Córdoba; ArgentinaFil: Daraio, M.C. Universidad de Buenos Aires. Hospital Privado; ArgentinaFil: Urrets Zavalía, Julio. Universidad Católica de Córdoba. Facultad de Ciencias de la Salud; ArgentinaFil: Urrets Zavalía, Enrique Alberto. Universidad Católica de Córdoba. Facultad de Ciencias de la Salud; ArgentinaFil: Gallino, N. Universidad de Buenos Aires. Hospital Privado; ArgentinaFil: Serra, Horacio M. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas; Argentina