Lung tissue from 221 definite and probable cases of malignant mesothelioma reported to the Australian Mesothelioma Surveillance Program from January 1980 through December 1985 and from an age-sex frequency matched control series of 359 postmortem cases were examined by light microscopic (LM) and analytical transmission electron microscopic (TEM) analysis and energy dispersive x-ray analysis (EDAX). Concentrations of total fibers (coated and uncoated) (LM), crocidolite, amosite, chrysotile, and unidentified amphibole (TEM) (fibers/g dry lung tissue) were measured. Fiber concentrations less than 10 microns in length and greater than or equal to 10 microns in length were separately quantified. By comparing cases (221) and controls (359 LM, 103 TEM), odds ratios for increasing fiber concentrations compared with less than 15,000 fibers/g (LM) and less than 200,000 fibers/g (TEM) (the respective detection limits) were calculated. Univariate analyses showed statistically significant dose-response relationships between odds ratio and fiber concentration for all fiber concentration measures. The relationship between log(odds ratio) and log(fiber concentration) was linear. Multiple logistic regression analysis showed that a model containing crocidolite greater than or equal to 10 microns, amosite less than 10 microns, and chrysotile less than 10 microns as explanatory variables best described the data. The odds ratios for a X10 increase in fiber concentration (fibers/micrograms) were as follows: crocidolite greater than or equal to 10 microns, 29.4 (95% confidence interval [CI], 3.6 to 241); chrysotile less than 10 microns, 15.7 (95% CI, 6.1 to 40); amosite less than 10 microns, 2.3 (95% CI, 1.0 to 5.3). An additive risk model gave similar results. In a subgroup of cases and controls with only chrysotile in the lungs, a significant trend in odds ratio with increasing fiber content was found.