To improve the digestibility of the forage crop alfalfa (Medicago sativa L.), cinnamyl alcohol dehydrogenase (CAD), which catalyses the last step in the biosynthesis of the lignin monomers, was down-regulated by using an antisense approach. A subset of six transgenic lines with reduced CAD activity and control lines were analysed when grown in the greenhouse and in the field. The down-regulation of the CAD enzyme was associated with a red coloration of the stem. The lignin quantity remained unchanged, but the lignin composition, as determined by thioacidolysis, was altered. The highest reduction of CAD activity was associated with a lower syringyl/guaiacyl (S/G) ratio and a lower S+G yield, mainly because of a decreased amount of S units. An increase in in situ disappearance of dry matter and of cell wall residue was detected in one of the transgenic lines grown in the greenhouse, and for two of the lines grown in the field the rate of disappearance of dry matter slightly improved. Furthermore, these two lines had a higher solubility in alkali as shown by the lower yield of saponified residue. This study opens perspectives for improving forage crop digestibility by the modulation of enzymes involved in lignin biosynthesis.