STUDY DESIGN: Randomised, double-blind, placebo-controlled crossover trial of melatonin supplementation to people with complete tetraplegia. OBJECTIVES: To investigate the effect that 3 mg melatonin supplementation has on objective and subjective sleep, quality of life and mood of people living with complete tetraplegia. SETTING: Austin Hospital Sleep Laboratory and participants' homes, Melbourne, Victoria, Australia. METHODS: Two week run-in followed by 3 week nightly administration of 3 mg melatonin or placebo, 2-week washout and further 3 week administration of the opposite treatment. Four testing sessions were conducted; the last nights of the run-in, treatment and washout periods. Testing sessions involved recording full polysomnography, completing a questionnaire battery and collecting urine and blood samples. The questionnaires assessed mood, sleep symptoms and health-related quality of life, and the urine and plasma samples assayed 6-sulphatoxymelatonin (aMT6s) and melatonin levels, respectively. A sleep diary was completed throughout the study. RESULTS: Eight participants (mean (s.d.): age 49.5 years (16), postinjury 16.9 years (7.1)) were recruited in which seven concluded the protocol. Endogenous-circulating melatonin was significantly higher (P < or = 0.01) following melatonin (urine: 152.94 μg h(-1) (74.51), plasma: 43,554.57 pM (33,527.11)) than placebo (urine: 0.86 μg h(-1) (0.40), plasma: 152.06 pM (190.55)). Subjective sleep improved significantly following melatonin specifically for duration of sleep per night and psychological wellbeing. Objective sleep showed a significant increase in light sleep with melatonin, with all other sleep parameters being unchanged. CONCLUSION: These results suggest that increasing melatonin in people with complete tetraplegia is beneficial, especially for subjective sleep. Investigation of the pharmacokinetics of melatonin metabolism in this population is warranted. SPONSORSHIP: This project is proudly supported by the Transport Accident Commission.