BACKGROUND: Toxicity, which is a key parameter in the evaluation of cancer treatments, can be underestimated by clinicians. We investigated differences between patients and physicians in reporting adverse events of androgen deprivation therapy (ADT) with or without docetaxel in a multicentre phase III trial in non-castrate metastatic prostate cancer. METHODS: The 385 patients included were invited to complete a 26-symptom questionnaire 3 and 6 months after the start of treatment, among which eighteen symptoms were also assessed by physicians, reported in medical records and graded using the Common Toxicity Criteria of the National Cancer Institute. Positive and negative agreements as well as Kappa concordance coefficients were computed. FINDINGS: Data were available for 220 and 165 patients at 3 and 6 months respectively. Physicians systematically under-reported patients' symptoms. Positive agreement rates (at respectively 3 and 6 months) for the five most commonly reported symptoms were: 61.0% and 64.3% hot flushes, 50.0% and 43.6% fatigue, 29.4% and 31.1% sexual dysfunction, 24.4% and 14.4% weigh gain/loss, 16.7% and 19.3% for joint/muscle pain. For symptoms most frequently reported as disturbing or very disturbing by patients, the clinicians' failure to report them ranged from 50.8% (hot flushes) to 89.5% (joint/muscle pain) at 3 months, and from 48.2% (hot flushes) to 88.4% (joint/muscle pain) at 6 months. INTERPRETATION: Physicians often failed to report treatment-related symptoms, even the most common and disturbing ones. Patients' self-evaluation of toxicity should be used in clinical trials to improve the process of drug assessment in oncology. FUNDING: French Health Ministry and Institut National du Cancer (PHRC), Sanofi-Aventis, Astra-Zeneca, and Amgen.