Mindfulness-based stress reduction (MBSR) is an 8-week training that is designed to teach participants mindful awareness of the present moment. In randomized clinical trials (RCTs), MBSR has demonstrated efficacy in various conditions including reducing chronic pain-related distress and improving quality of life in healthy individuals. There have, however, been no qualitative studies investigating participants' descriptions of changes experienced over multiple time points during the course of the programme. This qualitative study of an MBSR cohort (N = 8 healthy individuals) in a larger RCT examined participants' daily diary descriptions of their home-practice experiences. The study used a two-part method, combining grounded theory with a close-ended coding approach. The grounded theory analysis revealed that during the trial, all participants, to varying degrees, described moments of distress related to practice; at the end of the course, all participants who completed the training demonstrated greater detail and clarity in their descriptions, improved affect, and the emergence of an observing self. The closed-ended coding schema, carried out to shed light on the development of an observing self, revealed that the emergence of an observing self was not related to the valence of participants' experiential descriptions: even participants whose diaries contained predominantly negative characterizations of their experience throughout the trial were able, by the end of the trial, to demonstrate an observing, witnessing attitude towards their own distress. Progress in MBSR may rely less on the valence of participants' experiences and more on the way participants describe and relate to their own inner experience. Copyright © 2010 John Wiley & Sons, Ltd. Key Practitioner Message: This article • Analyses the ways in which participants in a mindfulness-based stress reduction (MBSR) clinical trial describe their experiences with mindfulness practice. • Carries out qualitative analysis of the ways in which participants' descriptions of home-based meditation practice contained in their practice diaries change over the course of an 8-week MBSR trial. • Demonstrates that the participants who successfully completed the 8-week course show a common developmental trajectory, as each participant used less reactive, judgemental language to describe their home meditative practice-based experiences by the end of the trial, even when, in the case of some participants, that experience was perceived as negative or distressing. • Suggests that progress in MBSR may rely less on the valence of participants' experience and more on the way participants describe and relate to their own inner experience.