TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHistorically, wellbore instability problems have been mainly attributed to the physico-chemical interaction between the rock and the drilling fluid, especially when this is a waterbased fluid (WBM). However, recent studies have shown that these chemical effects may be irrelevant in comparison with other energetic events which are present during the drilling operation of a well. Excessive drillstring vibration has been identified as such an event that can cause a great amount of damage to the wellbore. Although some studies present clear indications that the reduction of drillstring vibration has led, in many occasions, to an improvement on instability problems, still today this item only deserves attention from equipment divisions, aiming at reducing drillstring fatigue problems.The main objective of this paper is to show the clear existing relation between excessive drillstring vibrations and serious well instability problems. It is shown, in energetic terms, that drillstring vibration, when having enough lateral amplitude to hit the wall, is responsible for irreparable damage to the borehole. Simple ways of quantifying vibration effects in terms of energy are presented. The paper also presents field measurement data, obtained with surface vibration sensors, which clearly allow identification and correlation between vibration problems and wellbore instability, through the comparison of these vibration records with caliper logs.