Melatonin rhythms were assessed in 49 registered blind individuals by measurement of the urinary metabolite of melatonin, 6-sulfatoxymelatonin (aMT6s). Subjects had different causes of visual loss and were classified as having light perception or better (LP; n 5 19) or having no perception of light (NPL; n 5 30). Subjects collected four-hourly urine samples (eight-hourly overnight) for 48 h at weekly intervals for 3–5 weeks. The majority of LP subjects (14 of 19) had normally entrained aMT6s rhythms (mean acrophase range, 2.4–6.2 h), 4 were abnormally entrained to 24 h (mean acrophase range, 8.9–1.0 h), and 1 was unclassified. Conversely, mostNPLsubjects had abnormal rhythms (23 of 30), the incidence of which was greater in uni- and bilaterally enucleated subjects. The majority of NPL subjects (17 of 30) had free-running aMT6s rhythms (period range, 24.13– 24.79 h), 5 were abnormally entrained to 24 h (acrophase range, 7.2–20.6 h), and 1 was unclassified. Output (micrograms of aMT6s per 24 h) and amplitude (micrograms per h) of aMT6s production did not vary between LP and NPL subjects (mean 24-h output 6 SD, 12.7 6 7.5 and 9.4 6 6.4 mg aMT6s/24 h, respectively; mean amplitude 6 SD, 0.660.4 and 0.560.3 mg/h, respectively). These results indicate that a higher proportion of NPL subjects have abnormal melatonin rhythms compared to those with LP. (J Clin Endocrinol Metab 82: 3763–3770, 1997