The histamine content of tears of healthy sex- and age-matched subjects and patients affected by allergic or nonallergic inflammatory ocular diseases was determined through a new competitive reverse-phase high-performance liquid chromatography (HPLC) method. Tear samples from 50 healthy subjects, 30 patients affected by seasonal allergic conjunctivitis, 12 patients with bacterial conjunctivitis associated with Haemophilus influenzae and 8 patients with bacterial conjunctivitis associated with Streptococcus pneumoniae were analyzed for histamine concentration by o-phthaldialdehyde precolumn derivatization-based HPLC. In physiological conditions, the tear histamine content was low (2.26 ng/ml) and did not vary in relation to age and sex. Histamine levels were significantly higher in all the patients studied, to a greater extent in those affected by allergic (23.61 ng/ml) or Haemophilus influenzae-associated (21.53 ng/ml) conjunctivitis.