Background. Pulmonary fiber content of both asbestos and nonasbestos types were evaluated in Japanese patients with malignant pleural mesotheliomas. Methods. Pulmonary fiber content was analyzed in 16 patients and 16 case‐matched control subjects by transmission electron microscopy with energy‐dispersive X‐ray analysis using a low‐temperature ashing procedure. Results. The geometric mean content of total asbestos was significantly higher in the patients (22.0 × 106 fibers/g dry lung) than in the control subjects (2.24 × 106 fibers/g dry lung) (P < 0.01). When the asbestos content was analyzed by fiber type, the geometric means were also consistently and significantly higher among the patients compared with the control subjects (P < 0.01). Results were as follows: (1) amosite: patients 3.94 times 106 versus control subjects 0.23 × 106; (2) crocidolite: patients 3.56 times 106 versus control subjects 0.35 times 106; (3) total amphiboles: patients 16.0 times 106 versus control subjects 0.77 times 106; and (4) chrysotile: patients 3.76 times 106 versus control subjects 1.01 times 106. However, when individual total asbestos content was considered, 7 of the 16 patients (44%) had levels lower than the highest value noted among the control subjects. Pulmonary fiber content of patients and control subjects also revealed the presence of nonasbestos fibers. The geometric mean of nonasbestos fibers was significantly higher in the patients (87.3 × 106) than in control subjects (33.8 × 106) (P × 0.01). The major type of nonasbestos fibers in both groups was aluminum silicates. The mean of ratios of nonasbestos fiber contents to total asbestos contents in the patients and control subjects was 7.0 and 17.3, respectively. Conclusions. The results were mainly in agreement with the findings of earlier investigations, but fiber content of both chrysotile and nonasbestos fiber as well as those of amphibole asbestos were significantly higher in the patients than in the control subjects.