Technical Abstract: Outbreaks of Marek’s disease (MD) in vaccinated flocks still occur sporadically and lead to economic losses. Unfortunately, adequate methods to predict MD outbreaks are lacking. In the present study, we have evaluated if high load of challenge MDV DNA in peripheral blood could aid in the early diagnosis of MD and in monitoring efficacy of vaccines against MD. One experiment was conducted to simulate field conditions by combining various vaccines (HVT and HVT+SB-1) and challenge viruses (GA, Md5, and 648A). Vaccine efficacy among our experimental groups ranged from 13.3% to 94.2%. Each chicken was sampled three times during the length of the experiment (3, 5, and 15 weeks post challenge) and gross lesions were evaluated in chickens that died as well as at termination of the experiment. DNA was extracted from whole blood and buffy coats from each sample, and the load of challenge MDV DNA and HVT DNA were quantified by real time PCR. Chickens that developed MD by the end of the experiment had higher load of challenge MDV DNA (Ct GAPDH/ Ct gB ratios of 1.0, 1.04, and 1.05 at 3, 5, and 15 wpc, respectively) than those that did not developed MD (Ct GAPDH/ Ct gB ratios of 0.7, 0.69, 0.46). However, load of HVT DNA in blood was not correlated with the development of tumors (Ct GAPDH/ Ct HVT ratios from 0.04 to 0.10 in both groups). Vaccinated groups with more than 75% protection had statistically significant less challenge DNA virus (Ct GAPDH/ Ct gB ratios of 0.76, 0.70, and 0.45) than less protected groups (Ct GAPDH/ Ct gB ratios of 0.92, 0.97, and 0.85). No differences in the load of HVT DNA could be found between protected and non protected groups at any time point of the study (Ct GAPDH/ Ct HVT from 0.05 to 0.09 in both groups). Our results showed that load of challenge MDV DNA but not load of HVT DNA in blood can be used as criterion for early diagnosis of MD.