Background: Lymphoedema is a common late effect after breast cancer (BC) that has no effective cure once chronic. Accumulating evidence supports progressive strength training (PRT) as a safe exercise modality in relation to the onset and exacerbation of lymphoedema. In the ‘preventive intervention against LYmphoedema after breast CAncer’ (LYCA) feasibility study we examined the feasibility of a program of PRT in the first year after BC to inform a planned randomised controlled trial (RCT). Material and methods: LYCA was a one-group prospective pilot trial inviting women operated with axillary lymph node dissection for unilateral primary BC. Participants exercised three times a week for 50 weeks (20 weeks supervised followed by 30 weeks home-based exercise). The program ensured slow individualised progression during the exercise program. The primary outcome was feasibility measured by eligibility and recruitment rates, as well as questionnaire-assessed satisfaction and adherence to exercise. Furthermore, we assessed arm interlimb volume difference by water displacement, muscle strength by dynamic and isometric muscle testing and range of movement in the shoulder by goniometry. Results: In August 2015, eight of 11 eligible patients accepted participation. Two of them dropped out early due to other health issues. The remaining six participants had high exercise adherence through the supervised period, but only three maintained this through the home exercise period. Program satisfaction was high and no serious adverse events from testing or exercising were reported. One participant presented with lymphoedema at 50-week follow-up. Muscle strength markedly increased with supervised exercise, but was not fully maintained through the home exercise period. Range of shoulder movement was not negatively affected by the program. Conclusion: Recruitment, testing, and exercise in LYCA was safe and feasible. At the 50-week follow-up, there was one case of lymphoedema. The LYCA program will be further tested in a full-scale RCT.