Chronic breathlessness has devastating consequences. The minimal clinically important difference (MCID) for current intensity has been estimated as 9 mm on a 100 mm visual analogue scale (VAS). We aimed to determine MCIDs for commonly used dimensions and recall periods: the current unpleasantness and current, average, best and worst intensity of the last 24 h for chronic breathlessness.This was a secondary analysis of a randomised controlled trial of morphine versus placebo during seven days in people with chronic breathlessness from severe disease. The breathlessness scores were self-reported using a diary each evening on 100 mm VAS. The MCID for improvement in each score was estimated using anchor based and distribution based methods.283 participants (mean age 74.2 years; 63% males; 58% COPD; 87.0% mMRC 3-4) were included. Anchor-based MCIDs for breathlessness scores ranged from -13.9 mm to -9.5 mm. The MCIDs were similar when using different anchors and across all participants, and participants with more severe BREATHLESSNESS (mMRC 3-4), respectively. Distribution based effect sizes were: small (-4.7 to -6.3 mm), moderate (-9.4 to -12.5 mm) and large effect (-15.0 to -20.0 mm). Sample sizes for trials using the different scores were proposed. MCIDs of absolute change were more stable than using relative change from baseline.An improvement of about 10 mm on a 100 mm VAS is likely to be clinically meaningful across commonly used measures of chronic breathlessness (current intensity, unpleasantness, and average, best and worst intensity over the last 24 h) - to evaluate clinical benefit and effects in therapeutic trials.