The acid detergent lignin and Klason lignin methods were compared for their correlation with forage digestibility. Thirty-six forages, including C3 legumes and C3 and C4 grasses, were analyzed for sulfuric acid detergent lignin, Klason lignin, and in vitro digestibilities of dry matter (DM) and neutral detergent fiber (NDF). Twenty of these forages were also fed to lambs at restricted intake for measurement of DM and NDF digestibilities. Lignin concentrations determined by the two lignin methods were positively correlated, and the Klason lignin value was always greater than the acid detergent lignin concentration. The largest differences were observed for grass forages. Digestibilities of forage DM and NDF were negatively correlated with both lignin methods for the in vitro system and the lamb digestibility trials. The degree of correlation for the two lignin methods with digestibility was generally similar across all forages and within forage classes. Slopes of linear regressions of digestibility on lignin concentration did not differ between legumes and grasses. Although the sulfuric acid detergent lignin and Klason lignin procedures gave very different estimates of forage lignin concentration, they were similarly correlated with digestibility and should yield predictions of forage digestibility that have similar accuracy.