Although radiotherapy usually implies a cancer diagnosis, no available study has been reported as to what proportion of Thai patients treated with radiotherapy know their diagnosis and how they want related information revealed. We questioned 106 patients in order to determine the proportion who knew their cancer diagnosis, and interviewed patients who knew the diagnosis and relatives of patients both who knew and did not know it with semi-structured questionnaires concerning how they wanted to be told. Sixty-seven patients (63.2%) knew their cancer diagnosis. Multivariate analysis showed that the patients with the following characteristics were inclined to know their diagnosis: younger than 70 years old, head and neck or gynecological cancer, no previous treatment before radiotherapy, no accompanying relatives during the treatment, and patients with relatives who wanted to disclose the information. 97.0% of patients who knew the diagnosis indicated that they had really wanted to know, 89.5% and 73.3%, respectively of relatives who accompanied patients who knew and did not know it expressed the same wish. No demographic factors could predict who wanted to be told the diagnosis. The majority of patients and relatives who wanted the diagnosis disclosed wanted to know all related information before treatment. They wanted to hear this directly from the doctor in the presence of their relatives.