We read with interest the recent article by Lacasse et al1 (CHEST July 2012), which provided a comprehensive update on hypersensitivity pneumonitis (HP). We would like to comment, however, on the section relating to HP due to metalworking fluid (MWF) exposure and, in addition, highlight the experience in the United Kingdom in this area.2 Our group has recently published a review article summarizing the microbiologic and immunologic findings from all previously published MWF outbreaks.3 We have also investigated a number of MWF-HP outbreaks in the United Kingdom. In the most detailed of these, 17 workers met the case definition for MWF-HP,4 and in 14 of the 17 cases, the diagnosis was confirmed when reviewed by a panel of occupational respiratory disease experts.5 However, none of these workers (including one with a positive specific challenge to used MWFs from this workplace)4 or the other 112 workers tested had IgG precipitin responses to extracts of Mycobacterium immunogenum, chelonae, or fortuitum. In addition, mycobacteria could not be cultured from any of the 125 MWF samples taken from the workplace, and there was no evidence of mycobacterial DNA identified by polymerase chain reaction. Multiple MWF samples taken from two other workplaces with unrelated outbreaks of MWF-HP (C. M. Barton, MD, unpublished data) similarly showed no evidence of mycobacterial contamination or DNA.