Feeding of 20 to 30 million units of vitamin D for 3 to 8 days prepartum previously prevented about 80% of expected milk fever cases in mature Jersey cows with histories of milk fever. In this experiment vitamin D was fed continuously through the year via 32,000 units of vitamin D added to each .455 kg of concentrate fed (approximately 100,000 to 580,000 units/day). Milk fever incidence was measured after 5 yr of age (third calf) at 43 parturitions in mature Jersey cows with histories of milk fever and at 139 parturitions in mature Jersey cows with no previous milk fever. Continuous feeding of vitamin D reduced incidence of milk fever in cows with previous milk fever from 60% in the controls to 26.1% in the group fed vitamin D. In the cows with no milk fever previously, feeding vitamin D did not reduce incidence of milk fever (controls 23.7%, vitamin-D-fed 28.3%).