Observations of Si IV and O IV lines in solar active regions taken by the ultraviolet spectrometer and polarimeter on board the SMM are analyzed. The average frequency of UV bursts of all types is one every four minutes in a 28 arcsec by 28 arcsec field of view. About 80 percent of the bursts show an increased Si IV/O IV ratio, suggesting a density increase. The indicated electron densities are generally in the 10 to the 11th-12th range, with 16 percent indicating higher densities. Some of the more energetic bursts exhibit flarelike characteristics, and some of these show a secondary brightening which may be an analog of the soft X-ray gradual phase of flares. The high frequency of these UV bursts and their possible association with hard X-ray microflares and chromospheric evaporation may make them extremely important in the energetics of active regions and in the maintenance of their enhanced coronal emission. 23 references.