Interest in fatigue research has grown since the finding that fatigue/tiredness is the most frequently reported symptom of cancer and its treatment. But even though several authors have tried to conceptualize fatigue (Piper&Rieger, 1989; Cimprich, 1992; Gibson&Edwards, 1985; Winningham, 1994; Irvine et al. 1994; Grandjean, 1970; et al.), its mechanisms are still poorly understood. The aim of this study was two‐fold: i) to explore fatigue in cancer patients, inductively, and ii) to compare fatigue/tiredness experiences of healthy individuals with those of cancer patients to identify cancer‐specific fatigue/tiredness and related concepts. A qualitative research strategy was adopted using a grounded‐theory approach. The prospective study took place in the Oncology Department of the Kantonsspital St Gallen (Switzerland) with samples of 20 cancer patients and 20 healthy individuals. Unstructured, tape‐recorded interviews were conducted to collect data. Transcripts of the interviews were analysed using content analysis and constant comparison. Although different themes emerged between the two groups, both fitted a classification system that categorized expression of fatigue/tiredness as physical, affective or cognitive. Physical signs were more frequent than affective and cognitive signs in both groups. For the cancer patients, fatigue involved decreased physical performance, extreme, unusual tiredness, weakness and an unusual need for rest, which was distinctly different for healthy persons. Affective and cognitive distress were also more prominent in cancer patients. Interestingly, the concept of malaise was not identified by either sample and not understood as an expression of fatigue by this German‐speaking population. Linguistic differences in the description of fatigue/tiredness between healthy and ill individuals revealed different perceptions of the phenomenon. A step‐like theory, involving nociception, perception and expression of tiredness, was put forward tentatively to explain the production of fatigue/tiredness. The emerging concepts break tiredness/fatigue into expressions of physical, affective and cognitive tiredness/fatigue. The experience is different between healthy individuals and cancer patients. The generalization of data needs precaution but the results of the study identify and clarify ideas that might form an important basis for further, control