Forty-seven sources found in a X3.8-cm sky survey have been examined for variations in flux density. Over-all, 32% of the sources (15 out of 47) have been found to be variable. When sources which have the highest observed signal-to-noise ratio and which have been observed over more than 1200 days are con- sidered, the proportion of variable sources rises above 50%. Thus most of the sources with flat or inverted microwave spectra are rather rapid variables. Over the entire sky, there should be about 300 or more sources stronger than 1 f.u. at X3.8 cm which vary within a time period of only three years.