The Human Microbiome Project (HMP), started in 2008, aimed to characterize the microbial communities on the skin, in the oral and nasal cavities, and in the gastrointestinal and urogenital tracts. This project cross-sectionally described the microbial communities in a healthy human population, but did not adequately describe longitudinal stability. Out of the 236 adults included in the HMP, a second saliva sample was collected on a small subset (N=74) with a long interval between collections, an average of 203 days. Community similarity was calculated between two visits; for saliva samples, the Spearman correlation was high (rs=0.8). However, it was unclear which metric of community similarly was being used for this calculation. When we obtained the HMP data and calculated the Spearman correlation for specific diversity metrics in the saliva samples, the correlations were weaker and ranged from 0.225 to 0.524 for the Shannon Index and PD tree, respectively. Other studies with fewer people, but more time points, have had conflicting conclusions on the temporal variation of the oral microbiome. Given this variability, it is vital to establish whether a single measurement is representative of the typical composition of microbiota in a specific body site using more than two time points.