Background: There is now a body of evidence indicating a potential role for aspirin in colorectal cancer (CRC) prevention. In cardiovascular trials, effects on incidence of cancer metastases and short-term mortality suggest further possible roles in the treatment setting, supported by observational studies of aspirin use after cancer diagnosis. In the prevention setting, aspirin use has been limited by toxicity concerns, particularly of serious bleeding. In the adjuvant setting, benefits associated with reducing recurrence and subsequent treatment may outweigh these risks. The Add-Aspirin trial will investigate this, and will also consider possible mechanisms of action for aspirin effects, including the impact of PIK3CA mutations, where there are currently several theories and conflicting data. Methods: Add-Aspirin (ISRCTN74358648) is an international, phase III, double-blind, randomised, placebo-controlled trial recruiting patients who have undergone surgery and relevant adjuvant treatment for stage II or III CRC, as well as those with completely resected CRC liver metastases. Parallel randomised cohorts will address the question in breast, gastro-oesophageal and prostate cancer. Participants take aspirin 100mg daily for an 8-week run-in, to assess adherence and toxicity, and those suitable to proceed are randomised (1:1:1) to aspirin 100mg, aspirin 300mg or placebo daily for at least 5 years. A number of measures – including blood pressure control and PPI use where relevant - are in place to reduce bleeding risk. The primary outcome is disease-free survival (target hazard ratio = 0.8, n = 2600 in 5 years) with a long term analysis of survival planned across the tumour groups. Translational work includes a sub-study monitoring urinary thromboxane B2 as a marker of platelet activation in a subgroup (n = 500) to investigate mechanisms of action. Add-Aspirin opened in 2015 and recruited 1505 CRC patients during the first 3 years from 137 UK centres. 1282 (85%) proceeded to randomisation. A pre-planned feasibility analysis of run-in data (n = 2253 across all 4 tumour groups) provided reassuring data on safety, tolerability and adherence, and recruitment continues with centres in India and Republic of Ireland recently joining. Clinical trial information: 74358648.