The objective was to describe quarter-somatic cell count after occurrence of clinical mastitis (CM) in cows enrolled in negatively controlled randomized clinical trials. Cases of CM were enrolled in 2 separate randomized clinical trials conducted on 2 farms. In trial 1, cases confirmed as culture negative (NG) were randomly assigned to 5-d intramammary (IMM) ceftiofur therapy or non-treated control. In trial 2, cases confirmed as gram-negative (GN: E. coli or Klebsiella spp) were randomly assigned to 2 or 8-d of IMM ceftiofur therapy or non-treated control. Quarter milk samples were collected at detection of CM, 7, 14, 21, and 28 d after enrollment. Repeated measures analysis was performed to compare SCC trends among treatment groups. In trial 1, time (P < 0.01) had an effect on SCC compared with treatment (P = 0.11). In trial 2, pathogen (P < 0.01), treatment (P = 0.04), time (P < 0.01) and the interaction between pathogen-treatment and time (P < 0.01) had an effect on SCC. Quarter SCC at detection of CM was not different among pathogens. Quarter SCC decreased at different rates depending on the pathogen and treatment group. Proportion of cases with quarter < 200,000 cells/mL at 28 d after enrollment was not different within trials. For cases caused by Klebsiella spp., 8-d of IMM ceftiofur resulted in lower SCC but was not associated with proportion of cases with quarter-SCC < 200,000 cells/mL. Treatment was not associated with SCC for CM caused by E. coli or NG cases. We conclude that for non-severe CM, etiology has a greater influence on quarter SCC than IMM treatment using ceftiofur.