Margalit, BenLoeb, Abraham2019-10-032016Margalit, Ben, and Abraham Loeb. 2016. “Inferring the Distances of Fast Radio Bursts through Associated 21-Cm Absorption.” Monthly Notices of the Royal Astronomical Society: Letters 460 (1): L25–29. https://doi.org/10.1093/mnrasl/slw068.1745-39251745-3933http://nrs.harvard.edu/urn-3:HUL.InstRepos:41461250The distances of fast radio burst (FRB) sources are currently unknown. We show that the 21-cm absorption line of hydrogen can be used to infer the redshifts of FRB sources, and determine whether they are Galactic or extragalactic. We calculate a probability of similar to 10 per cent for the host galaxy of an FRB to exhibit a 21-cm absorption feature of equivalent width greater than or similar to 10 km s(-1). Arecibo, along with several future radio observatories, should be capable of detecting such associated 21-cm absorption signals for strong bursts of greater than or similar to several Jy peak flux densities.en-USJournal Article2019-10-0310.1093/mnrasl/slw068