Breeding of alfalfa (Medicago sativa L.) for resistance to Sclerotinia crown and stem rot (SCR; causal agent Sclerotinia trifoliorum Eriks.), which is a serious disease in Japan, was started in 1983. Increase of resistance to this disease has been examined following repeated selection of surviving plants from artificially inoculated field plots to the 9th generation. The strains selected for SCR resistance showed a higher resistance than Natsuwakaba and Tachiwakaba which were used as breeding materials and control cultivars. The effectiveness of selection based on the survival rate of progenies increased gradually as generations in which the recurrent selection was performed advanced. Realized heritability was low (h2=0.078) in the 1st and the 2nd generations in which selection was performed but was high (h2=0.364) in the 3rd to 9th generations. It is considered that the selection effect on SCR resistance became higher in the later generations, probably due to the accumulation of genes which have minor or polygenic effects on resistance.