The incidence of malignant melanoma was highest among those who had flown routes that extended over five time zones indicating that disturbance of the circadian rhythm and thus the homeostasis of melatonin may play a part in the aetiology of melanomas. Melatonin has an oncostatic action and has been tried in the treatment of metastatic melanoma.34 35 The small size of our study does not allow us to separate between the theoretical synergism, or additive effect, of cosmic radiation, disturbed circadian rhythm, and the potential confounding of possible excessive sunbathing.