A therapeutic trial of 1% indomethacin (Indoptic) eye drops was carried out in 21 children. Looking for possible mediators of inflammation in vernal conjunctivitis, prostaglandin E₂ (PGE₂) and leukotriene B₄ (LTB₄ levels in the tears of 9 patients were measured and the effect of the treatment on them examined. A control group of 10 unaffected children was added. Out of 42 eyes in which indomethacin treatment was instilled, only 17 remained in treatment through a 6-week follow-up period. In a few of them a moderate improvement was obtained. The mean level of PGE₂ in the patients before treatment was found to be slightly lower than that in the control group, and it dropped even lower during treatment. The average LTB₄ level found in patients before treatment was significantly higher than the control group; it decreased somewhat following treatment, but not significantly. This is the first report of elevated LTB₄ levels in vernal conjunctivitis, previously not recorded in the literature, it points to the possible role of LTB₄ in the pathogenesis of the disease. A constant relationship was observed between low PGE₂ levels and high LTB₄ content in the patients' tears during highly inflamed states of the eye. We conclude that: (a) indomethacin did not prove to be a highly effective topical treatment for vernal conjunctivitis; (b) PGE₂ does not seem to be a dominant mediator of inflammation in this disease; and (c) LTB₄, on the other hand, apparently has a role in the mechanism of inflammation of the disease, thus raising hopes for future addition to therapy.