Since 1986 chestnut blight, caused by Cryphonectria parasitica is spreading in the small isolated chestnut (Castanea sativa Mill.) stands north of the Swiss Alps. Affected are coppice stands, rejuvenated old orchards and trees in parks. At all sites diversity of the vegetative compatibility (vc) types of C. parasitica is low (1-3 vc types). The release of the hypovirus CHV-1, therefore, seemed to be promising. At 17 sites in 5 states, altogether 448 cankers were treated. Local C. parasitica isolates containing a Swiss CHV-1 strain were used. Holes were applied around cankers on sprouts and branches that were reachable from the ground, and filled with the hypovirulent inoculum. One to two years later the canker types were assessed and C. parasitica was re-isolated from treated and untreated cankers. The comprised data show that the treatments were successful. In one state, where CHV-1 was applied repeatedly, over 50% of all cankers - treated and untreated - were assessed as healing. In 3 states significantly more cankers were healed among the treated cankers (32 to 33%) compared to the untreated (0 to 20%). Only in one state the percentage of healing cankers remained inexplicably low (12% for treated and 23% for untreated cankers). CHV-1 infected C. parasitica isolates were obtained from 33 to 75% of the treated cankers. Five to 35% of the untreated cankers also yielded CHV-1 infected C. parasitica isolates, an indication that CHV-1 is not only persistent in treated cankers but is also spreading slowly to new cankers. Difficulties of application of CHV-1 and conditions for a sustainable biocontrol are discussed.