A study of the correlation between cotton dust and gram negative bacterial endotoxin concentrations in yarn preparation areas in cotton mills was conducted. Vertical elutriator samples were collected in the opening, cleaning, carding, drawing, roving, and spinning areas of two cotton mills in Shanghai, China. Samples were also collected in a silk mill for comparison. The samples were analyzed for cotton dust by a gravimetric assay and for endotoxin by the Limulus amebocyte assay. Time weighted average (TWA) cotton dust concentrations ranged from 0.14 to 3.75mg/m3 in mill 1 and from 0.04 to 2.93mg/m3 in mill 2. The highest median dust concentration in mill 1, 1.69mg/m3, occurred in the opening area. The highest median dust concentration in mill 2, 1.73mg/m3, occurred in the cleaning area. TWA endotoxin concentrations ranged from 15.3 to 1,462 nanograms per cubic meter (ng/m3) in mill 1 and from 1 to 1,697ng/m3 in mill 2. The highest median endotoxin concentration in mill-1, 478ng/m3, occurred in the carding area. The highest median endotoxin concentration in mill 2, 750ng/m3, occurred in the drawing area. Median dust and endotoxin concentrations in the silk mill were 0.12mg/m3 and less than 0.1ng/m3. Cotton dust and endotoxin concentrations were significantly correlated overall at both mills. When analyzed according to work area, the correlations were still strong except in the carding area of mill 1. Linear regression models predicted that for a cotton dust concentration equal to the current standard, 0.2mg/m3, the endotoxin concentrations in mill 1 and mill 2 would be 56 and 68ng/m3, respectively. The authors conclude that correlations between dust and endotoxin concentrations in a typical cotton mill may be stronger than those observed in experimental card room studies. Cotton dust concentrations may be a suitable index of endotoxin exposure of workers employed in the cotton textile industry throughout the world.