Viticulture in the Mediterranean region has been improved by agronomic methods based on ecophysiological and genetic knowledge of the species and varieties cultivated. Plant growth, yield and quality are highly dependent on climate. Grape sugar content and wine alcohol content are considered as important quality parameters. The objective of our study is to analyse the effects of the current global change on the percentage of alcohol by volume (ABV) in red wines from vineyards located in the Montsant Designation of Origin (DO) (Priorat County, north-eastern Spain). We present an annual series of percentage of ABV in red wines over the 1984-2008 period (25 years), which is one of the longest series of this viticultural variable in Spain. We do not detect any significant trend of alcohol levels in red wines from the Montsant DO along the 1984-2004 subperiod, but a sharp increase about 1% (by volume) is observed after 2004; we statistically checked that the last four years constitute an outlier period in the series. We consider climate evolution over the 1984-2004 period in the study area in order to find some relation with alcohol levels in red wines. Agronomic practices and land cover changes are also taken into account. Results show that the interannual variability of the alcohol levels in red wines are partially explained by temperature and precipitation conditions few days before the vintage. The high percentages of ABV since 2005 may be associated with new trends in viticulture techniques rather than with climate change.