ObjectiveTo explore physically inactive breast and colon cancer patients’ prediagnosis exercise history and attitudes to physical activity (PA) and experiences in initiating PA while undergoing adjuvant chemotherapy.DesignAn explorative qualitative study guided the interpretive analysis of semistructured, open-ended interviews conducted at initiation of chemotherapy and after 12 weeks. The study was embedded in a pilot randomised controlled trial.SettingParticipants were recruited from the Oncological Department at a hospital in Copenhagen, Denmark.Participants33 patients with cancer, median age 49 years: 25 patients with breast cancer and 8 with colon cancer, 72% with a low cardiac respiratory fitness level and the majority with a high level of education. Patients received adjuvant chemotherapy, oncologist’s PA recommendation and exercise, cancer nurse specialist’s counselling prior to allocation to PA interventions or waitlist control group.ResultsPrediagnosis exercise had been excluded from patients’ daily lives due to perceptions of exercise as boring, lack of discipline and stressful work conditions for both genders. Recommendations from oncologists and nurses inspired the patients to reconsider their attitudes and behaviour by accepting recruitment and participation in PA interventions during chemotherapy. Despite extensive side effects, most patients adhered to their PA commitment due to their perception of the bodily, emotional and social benefits and support of healthcare professionals, peers and family.ConclusionThe patients’ attitude towards exercise transformed from having no priority in patients’ daily lives prediagnosis to being highly prioritised. This study identified four important phases in the exercise transformation process during the patients’ treatment trajectory of relevance to clinicians in identifying, motivating and supporting physically inactive patients with cancer at long-term risk. Clinicians should address young, highly educated patients with cancer at onset of adjuvant chemotherapy due to their specific risk of a sedentary lifestyle resulting from being in stressful, ambitious careers.Trial registration numberCurrent Controlled Trials (ISRCTN24901641), Stage: Qualitative results.