Few studies have investigated the efficacy of extended ceftiofur therapy and none have focused on extended therapy for naturally occurring clinical mastitis. The objective of this study was to compare the efficacy of extended intramammary ceftiofur therapy of 8 d duration with a standard 2-day regimen for the treatment of naturally occurring mild to moderate clinical mastitis in lactating dairy cows. Holstein cows from 22 dairy herds (n = 241) were randomly allocated to the 2 treatment groups. For each case of mastitis, 125 mg of ceftiofur hydrochloride was administered intramammary once a day for 2 or 8 d. Clinical cure, 21 d after the last treatment, was 89% (98/110) in each group. Bacteriological cure 21 d after the last treatment for the 2- and 8-day regimens were 32% (15/47) and 61% (25/41), respectively, for all bacteria (P = 0.007), 64% (9/14) and 82% (9/11), respectively, for streptococci (P = 0.50), and 0% (0/20) and 47% (9/19), respectively, for Staphylococcus aureus (P = 0.0004). There were no statistical differences between groups for new intramammary infections. Overall, ceftiofur extended therapy increased cure when compared to a 2-day regimen for the treatment of naturally occurring mild to moderate clinical mastitis in lactating dairy cows.