Over a six-year period from 1979 to 1985 the incidence of mastitis among dairy herds being recorded in a veterinary practice in Gloucestershire decreased from 26.5 per cent to 19.6 per cent of cows affected each year and from 51.0 to 31.7 cases per 100 cows per year. Over the same period the rolling mean herd milk cell count fell from 346,000/ml to 243,000/ml and the usage of intramammary antibiotic tubes fell from 2.6 to 2.1 per lactating cow per year. The proportion of cases needing repeat treatments in a 12-month period also fell from 25 per cent to 10.6 per cent. Possible causes for the decrease in the incidence of mastitis are given. The cost of an average case of mastitis was estimated to be 40 pounds, and on this basis the farms involved reduced their losses from mastitis by 772 pounds per 100 cows per year, a more than 12-fold return on their investment in veterinary services.