For an epidemiologist, one of the most important questions still to be answered is: Does byssinosis lead to permanent, irreversible changes in lung function? A more general question is of equal importance: Does exposure to organic textile dusts lead to permanent respiratory disability? Questions relating to the aetiological factor or factors involved, and questions as to biological mechanisms are clearly of relevance to the control of the acute manifestations of byssinosis in exposed workers but the question of irreversible effects is of fundamental relevance to any overall evaluation of byssinosis, and to our assessment of the danger of organic textile dusts to workers including those who do not manifest the acute symptoms of byssinosis. Questions relating to long-term effects will not however be easily answered.