Within the substantial body of research examining the professional knowledge of physical education (PE) teachers one particular area remains relatively under-explored: namely, their understandings of young people's participation in leisure-sport and the implications of this, if any, for the practice of PE. There are grounds for thinking, however, that in this aspect of their professional knowledge PE teachers might not be as conversant with patterns of participation--among young people, generally, and their own pupils, in particular--as one might expect. In order to examine this tentative hypothesis, the present study involved focus groups with a total of 29 PE teachers at six secondary schools in England. A central finding of the study was that PE teachers' perceptions of their youngsters' leisure-sport lives tended to be characterized by a blend of myth and reality. Many teachers, for example, underestimated the levels of participation in leisure-sport both of their own pupils and the 15-16 years age group, generally. Nevertheless, the teachers' observations regarding what amounted to growing and diversifying sporting repertoires among their pupils were, to a greater or lesser degree, commensurate with the profiles reported by the pupils, and with wider trends associated with the changing lifestyles and preferences of young people. The paper concludes by briefly locating this study of professional knowledge within the sociology of knowledge, while observing that the content and form of PE for Year 11 pupils at the six schools in this study appeared to be informed by the common-sense, everyday knowledge of PE teachers rather than by evidence from national or local surveys of young people or studies of their own pupils. (Contains 5 notes.)