Scientific papers estimating economic losses from mastitis were reviewed. Reduced milk production from cows with subclinical mastitis was responsible for the largest losses. Losses from mastitis in the United States in 1976 were estimated by usable responses to a survey from 33 states representing 9.5 million cows or 86% of the dairy cow population. Losses of milk yields caused by mastitis were 386 kg/cow per yr and losses of discarded milk 62 kg/cow per yr. Annual losses per cow from mastitis were a) reduced milk production, $81.32; b) discarded milk, $12.88; c) cost of veterinary services, $1.97; d) cost of drugs, !3,86; e) increased labor, $2.28; f) decreased sale value, $5.72; g) increased replacement costs, $9.32; h) total, $117.35. For the 11 million cows in the United States 1976 losses from mastitis were $1.294 billion. Mastitis research was supported publicly at 22 locations under 43 projects. Approximately 24 scientist years were involved and $2.7 million of public funds were expended annually. If economic losses from mastitis were reduced 2% per year for 10 yr by research and expenditures for mastitis research remained the same, the benefit to cost ratio from mastitis research would be approximately 9.6 to 1.