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 PETRO-BRAS 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.