Milk samples were collected from 108,312 dairy cows during 1601 farm visits made between January 1991 and June 1995. The herd visits were made by personnel from the Central Laboratory of the Quality Milk Promotion Services at Cornell University (Ithaca, NY) to farms located in central New York and northern Pennsylvania. Dairy Herd Improvement Association records were available for 32,978 cows in 327 herds. Intramammary infections, as defined by positive milk cultures, were present in 48.5% of all cows and in 36.3% of cows in herds enrolled in the Dairy Herd Improvement Association. Over 75% of the intramammary infections were caused by Streptococcus agalactiae, Streptococcus spp. other than Strep. agalactiae, Staphylococcus aureus, and coagulase-negative staphylococci. Mean days in milk at the time of diagnosis, linear score of the somatic cell count, cost of milk loss per lactation, and milk production effects were calculated for 24 etiologic agents of bovine mastitis.