This study aimed to acquire information about the knowledge patients have about radiation and radiotherapy, and the sources and impact of this knowledge. Data collection was by semi-structured interviews with 30 randomly selected adult patients conducted during the first half of a course of radiotherapy. Data analysis used three major themes: language, understanding and sources of knowledge. Few interviewees had any idea of the physical characteristics of radiation treatment, or they found it difficult to explain. Knowledge about radiation use, other than for medical purposes, most commonly concerned the atom bomb. Patients understood radiotherapy in terms of its action on cancer. Almost all offered a sensible description of why radiotherapy was being used to treat their cancer. Most talked about radiation attacking their cancer in some way and many worried about its damaging effect. A third gave detailed information about what they had become aware of through the written press, or radio or television. The message taken from press reports was largely negative.