Variation in trunk diameter (TDV) has been proposed as an indicator of the water status of grape vines, with apparently contradictory results. In Vitis vinifera L. var. Crimson Seedless we evaluated the water potential (Ψ), an indicator normally used to determine the water status of grapes, and TDV, comparing two irrigation treatments which restored 100% or 50% of the ETc, to compare the sensitivity of these two indicators to a decrease in soil water. Two evaluation periods of 23 d each were used, the first during the exponential growth phase and the second in the post-veraison period, when the trunk stops growing. In both periods TDV showed coefficients of variation greater than 25%, compared to the 7-10% recorded for water potential. However, during the first measuring period (82-105 d after bud break) the TDV of the treatment with water deficit decreased by more than 30%, while water potential only decreased from 6-12%. In the second measurement period (112-155 d after bud break) in which fruit growth is predominant, the control showed TDV values 100% greater than those of the treatment with water deficit, while the differences in water potential were between 10% and 13%. These results demonstrate the sensitivity of the continuous variation of the trunk diameter and the utility of TDV as a criterion for irrigation control, taking into consideration the development stage of the vines.