OBJECTIVE:To identify whether slow aquatic exercise in the form of modified Ai Chi is more effective than conventional (faster pace) aquatic therapy at reducing arm volume in women with or at risk of breast cancer related lymphoedema. METHODS:Randomized, cross-over controlled trial with concealed allocation and blinded assessment. Eighteen women with a history of breast cancer related lymphoedema were recruited. Participants received two intervention sessions (randomized order) with one week apart. Interventions were a 50min conventional aquatic intervention or a 50min modified Ai Chi. Arm volume was measured as the difference between affected and unaffected arm; bio-impedance was measured as an index of extracellular fluid; satisfaction was measured via a 12 question form. Outcomes were measured before, immediately after and one hour after intervention. RESULTS:Comparison between interventions showed larger decreased arm volume of 140mL (95%CI 17-263) immediately after intervention in favor of the Ai Chi intervention, however it was not sustained at 1h follow-up. A post hoc analysis showed 72% of participants had a decrease in arm volume immediately after Ai Chi compared to 28% immediately after conventional aquatic therapy; with a number needed to treat of 3 (95%CI 1.4-6.6). There were no differences between interventions for bio-impedance. Satisfaction was good for both interventions. CONCLUSION:Slow pace aquatic exercise is more effective than conventional aquatic exercise immediately after intervention for arm volume. Also, undesirable increase in arm volume seems to subside after 1h, which can be beneficial if therapy does not address arm volume. TRIAL REGISTRATION:ACTRN12614000557639 (https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=12614000557639).