A five year prospective study, documenting respiratory symptoms, measurement of lung function and assessment of airborne dust concentrations, was performed in eight Lancashire textile mills. Two mills processed man-made fibre, two processed a cotton blend, two medium grade cotton and two mills processed waste cotton. Each year, operatives were administered a respiratory questionnaire and invited to perform spirometry, measurements of FEV₁ and FVC were recorded. Work area dust concentrations were assessed by following the Health and Safety Executive EH25 guideline on static dust sampling and dust concentrations within the personal breathing zone were assessed by operatives wearing personal dust samplers over a working shift. In total, 1611 operatives have taken part over the five year study period. For the purpose of this analysis, the data set has been stratified. In total 1394 operatives were included in this analysis, 569 operatives in man-made fibre, 202 operatives processing blend cotton, 413 operatives processing medium grade cotton and 210 operatives processing waste cotton. Both measures of lung function decline were low for all four fibre groups; Man-made fibre FEV₁ decline = -17mls/yr FVC = -11mls/yr, Blend FEV₁ = 4mls/yr FVC = 4mls/yr, Medium FEV₁ = -5mls/yr FVC = -5mls/yr, Waste FEV₁ = -21mls/yr FVC = -20mls/yr. However, man-made fibre and waste processing operatives exhibited greater rates of declines in lung function. Within the man-made fibre group over half of the population have had previous exposure to cotton and this may be reflected in the measures of lung function decline. Within the cotton group lung function decline was greatest in the high dust exposure group, waste cotton workers, suggesting a dose response relationship. Byssinosis was associated with increased cumulative exposure to dust and length of time working in the cotton. Operatives with byssinosis also had lower baseline measures of lung function and greater rates of lung function decline.