OBJECTIVE To determine the effectiveness of the in vivo administration of vitamin E as treatment for reactive oxygen species-associated male infertility. METHODS University-based center for reproductive medicine. METHODS Double-blind randomized placebo cross-over controlled trial. METHODS Thirty healthy men with high levels of reactive oxygen species generation in semen and a normal female partner. METHODS Patients were allocated to two groups according to the blinded randomization. Each patient received either 600 mg/d of vitamin E (Ephynal, 300 mg tablets; F. Hoffman-La Roche Ltd., Basle, Switzerland) (order A) or identical placebo tablets (order B) for 3 months. Then after a 1-month wash-out period the patients were crossed-over to the other treatment. METHODS Improvement in the in vitro function of the spermatozoa measured by conventional semen analysis, computerized motility assessment, determination of reactive oxygen species generation, binding to the zona pellucida of the unfertilized human oocyte in a competitive zona binding assay, development of hyperactivated motility (both spontaneous and in the presence of 20% of the natural agonist, human follicular fluid) and pregnancy. RESULTS Rise in the blood serum vitamin E levels after treatment accompanied by improvement in one of the sperm function tests: the zona binding assay. The zona binding ratio for order A improved from 0.2 (range 0 to 0.5) before treatment to 0.5 (range 0.1 to 1.0) after treatment, the corresponding values for order B were 0.2 (range 0 to 1.0) before treatment and 0.3 (range 0.1 to 0.7) after treatment. CONCLUSIONS Oral administration of vitamin E significantly improves the in vitro function of human spermatozoa as assessed by the zona binding test.