Rat heart and liver cDNAs for precursor of l-3-hydroxyacyl-CoA dehydrogenase have been cloned and sequenced. The results indicate that these different rat organs express identical dehydrogenases. Furthermore, pig heart mRNA for l-3-hydroxyacyl-CoA dehydrogenase precursor was amplified by reverse transcription–polymerase chain reaction, and all the cDNA clones were found to encode a precursor of liver l-3-hydroxyacyl-CoA dehydrogenase (X.-Y. He, S.-Y. Yang, Biochim. Biophys. Acta 1392 (1998) 119–126) but not the well-documented heart form of the dehydrogenase (K.G. Bitar et al., FEBS Lett. 116 (1980) 196–198). Sequencing data and other evidence establish that the pig, like the rat, has the same dehydrogenase in heart and liver. Since the size and structure of pig heart l-3-hydroxyacyl-CoA dehydrogenase are identical to the pig liver dehydrogenase, reports that relied on the published sequence of the pig heart dehydrogenase need to be re-evaluated. For example, the signature pattern of the l-3-hydroxyacyl-CoA dehydrogenase family is HXFXPX₃MXLXE. Furthermore, the published crystal structure of the pig heart dehydrogenase that substantiated each subunit comprising 307 residues with a mercury-binding residue at position 204 (J.J. Birktoft et al., Proc. Natl. Acad. Sci. U.S.A. 84 (1987) 8262–8266) must be re-examined in accordance with this revelation.