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The knowledge of aridity is needed to explain landscape characteristics and the rational utilization of water resources. With global warming, an increase in aridity is expected for many parts of the world. This study was undertaken to analyse monthly and annual aridity index (IA), utilizing ratio of precipitation (P) over reference evapotranspiration (ETo), at ten stations located in critical agricultural regions in Iran. The Kendall and Spearman tests with considering serial correlation effect were used for analysis. An increase in aridity, as the P/ETo index decreased, was found during the study period, and the increase was more obvious in the semi-arid region of Iran than in the humid region. The increase of aridity was caused by the concurrent occurrences of negative P trends and positive ETo trends. The results indicated the lowest numbers of the negative IA trends in the July series and the highest numbers in the January, February and March series. According to the TheilSen’s estimator, the annual P of the semi-arid and humid regions decreased at the average rates of (-)22.87 and (-)6.24 mm year-1 per decade respectively, whereas the annual ETo values in the semi-arid and humid regions increased by (+)11.42 and (+)6.82 mm year-1 per decade, respectively. Copyright (C) 2012 Royal Meteorological Society