Although melatonin treatment has been shown to phase shift human circadian rhythms, it still remains ambiguous as to whether exogenous melatonin can entrain a free-running circadian system. We have studied seven blind male subjects with no light perception who exhibited free-running urinary 6-sulphatoxymelatonin (aMT6s) and cortisol rhythms. In a single-blind design, five subjects received placebo or 5 mg melatonin p.o. daily at 2100 h for a full circadian cycle (35-71 days). The remaining two subjects also received melatonin (35-62 days) but not placebo. Urinary aMT6s and cortisol (n=7) and core body temperature (n=1) were used as phase markers to assess the effects of melatonin on the During melatonin treatment, four of the seven free-running subjects exhibited a shortening of their cortisol circadian period (tau). Three of these had taus which were statistically indistinguishable from entrainment. In contrast, the remaining three subjects continued to free-run during the melatonin treatment at a similar tau as prior to and following treatment. The efficacy of melatonin to entrain the free-running cortisol rhythms appeared to be dependent on the circadian phase at which the melatonin treatment commenced. These results show for the first time that daily melatonin administration can entrain free-running circadian rhythms in some blind subjects assessed using reliable physiological markers of the circadian system.