Background: In lung transplant recipients (LTRs), tacrolimus is often utilized as a core component of immunosuppressive regimens. Although tacrolimus can be delivered orally or intravenously, oral tacrolimus is associated with fewer adverse effects. Various reports have suggested that sublingual tacrolimus may be used as an alternative to oral tacrolimus; however, information regarding converting between routes is limited. We aimed to identify a dose conversion ratio between oral and sublingual tacrolimus in LTRs. Methods: We identified adult LTRs at the University of California, San Francisco, who transitioned between oral and sublingual tacrolimus between 2005 and 2010 (n = 34). For tacrolimus, we obtained steady-state blood concentrations and total daily doses before and after the route conversion. Blood concentrations divided by daily doses were calculated for each route. The conversion ratio was then defined as: (blood concentration sublingual/daily dose sublingual)/(blood concentration oral/daily dose oral). This ratio was tested in inpatient vs outpatient settings and in the presence of impaired gastric emptying. Adverse effects, including nephrotoxicity, hepatotoxicity and anaphylaxis, were evaluated. Results: The conversion ratio of sublingual to oral tacrolimus was 0.46 ± 0.20 (mean ± SD). The ratio was not associated with hospital setting (p = 0.82) or with impaired gastric emptying (p = 0.31). When comparing sublingual to oral tacrolimus administration, there were no differences in serum creatinine, liver function tests or anaphylaxis. Conclusions: Tacrolimus administered sublingually at approximately half of the oral dose achieves therapeutic blood concentrations and is safe in LTRs. Delivery via the sublingual route using this conversion ratio may aid clinicians in maintaining therapeutic tacrolimus blood concentrations while avoiding the need for intravenous administration. © 2012 International Society for Heart and Lung Transplantation. All rights reserved.