Wellbore instability can be attributed to several causes. The ones thought to be most important include: chemical interaction with the drilling fluid, high tectonic stresses, and insufficient mud weight. Drillstring vibration, although not traditionally addressed as a potential cause, might influence the stability of wellbores drilled in specific formations. Evidence of the strong correlation between severe vibration and wellbore instability has been reported in the literature. However, a more thorough understanding of the phenomenon is still lacking. This paper describes a study that has been developed by PETROBRAS focusing on how drillstring vibration impacts wellbore instability. Vibration has been monitored in some wells, and events related to borehole enlargement were observed. Four field cases are presented showing a strong correlation between high vibration level and wellbore enlargement in different lithologies. Other sources of wellbore enlargement have also been identified, and they can be clearly separated from vibration.