American Indian children with mild chronic trachoma were treated with doxycycline, 2.5 to 4 mg/kg of body weight, given as a single daily dose on five days each week, for a total of 28 doses given in 40 days. The study was carried out double-blind, with randomized placebo controls. Detailed ophthalmologic evaluation continued for five months after the treatment course. The trachoma in the doxycycline-treated children improved markedly, as compared to those receiving placebo (P <.001 at 20 weeks after treatment). Few untoward drug effects were observed. Serum levels of the drug were maintained through much of the treatment period, but drug levels in tears were low and irregular. This one-dose-a-day therapy deserves consideration for mass treatment of trachoma.