The presence or absence of tumor occurrence at final observation is shown in Table 1. Tumor occurrence was most frequently observed in the lungs for all groups at 15 of 77 (19.5%) animals, but there was no significant difference between groups. One case in the vehicle group developed mesothelioma that was thought to originate from the pleura (Figure 3A and andB),B), but all other cases were adenomas (Figure 3C and andD).D). Although we checked for the presence of thin- and thick-MWCNTs in cases with tumors that occurred in the lungs, we found no evidence of MWCNTs.