To explore changes in melatonin secretion patterns and biologic rhythms in Parkinson's disease patients with or without levodopa-related motor complications (LDRMCs), the authors investigated, in an observational study, circadian rhythms of central temperature, motor activity, plasma cortisol, and melatonin in three groups: de novo untreated patients (group I), patients treated with levodopa + dopamine agonist and without LDRMCs (group II), and patients treated with levodopa + dopamine agonist and with LDRMCs (group III). There were no differences among the three groups for the rhythm of temperature, motor activity, or plasma cortisol. There was a significant (p < 0.05) phase advance in plasma melatonin secretion in patients receiving a dopaminergic treatment compared with untreated patients. The daytime area under the curve (AUC) was increased significantly in group III, and the nighttime AUC-to-daytime AUC ratio of melatonin secretion decreased significantly in group III, suggesting that the nychthemeral pattern of melatonin secretion was changed in patients with LDRMCs. Comparison of the three groups suggests a slight but insignificant phase advance and amplitude decrease of circadian melatonin secretion related to both evolution and treatment of Parkinson's disease. Despite the lack of a global desynchronization in other circadian biologic rhythms, the circadian secretion pattern of melatonin is modified in patients with LDRMCs.