The antibiotic sensitivity profile of Escherichia coli bacteria isolated from cases of mastitis in cattle matches that of commensal E. coli. Although there are significant differences between studies in antibiotic sensitivity, there is no evidence that mastitis occurrence or severity is related to antibiotic resistance. Almost all E. coli isolated from mastitis cases are susceptible to two or more antibiotics authorised for the treatment of cattle. However, although we have effective antibiotics in vitro, the in vivo evidence is not so clear-cut. For mild-to-moderate cases, there is only limited evidence that antibiotics are beneficial; for acute cases, there are more studies that show a benefit but also more studies that show there is no benefit from antibiotic treatment. On balance, treatment of acute E. coli mastitis with parenteral antibiotics is probably indicated, but antibiotic treatment should be part of a package alongside non-steroidal anti-inflammatory drugs and fluids.