The summer temperature variations over the Mediterranean Basin were studied through the 850 hPa level for the months June--August along the period 1948--2003, based on the NCEP-NCAR CDAS-1 archive. The most prominent feature found is warming, larger than the global value, over the majority of the study region with a maximum of 0.04 Ky-1 over Sicily. At the same time, cooling was noted over Algeria and the Balkans. The trend for the 10% upper quantile of the days over the warming region was found larger than the seasonal, reaching 0.053 Ky-1, implying that heat waves lead the general long-term trend. The shape of the long-term curve was found to fit the global one over the warming region. No synoptic-dynamic factor was found to account for the intensity and spatial distribution of the warming trend. This, together with the fit of the trend with the global one, suggests that the Mediterranean Basin manifests the increase in the greenhouse effect in the summer season.