Contrary to a widespread assumption, deuterium is not simply destroyed in stars: deuterium is also synthesized in the atmospheres of active stars. This nonprimordial synthesis of D arises when protons accelerated in flares interact with the atmosphere, create a flux of free neutrons, and these neutrons then undergo radiative capture on atmospheric protons. Radiative capture does not result in excess production of Li, Be, or B. Ejection of flare-processed material contaminates the interstellar medium (ISM), as was originally suggested by Coleman & Worden. Estimates of the amount of flare-created D are subject to considerable uncertainties, but we find, using stellar parameters within permitted ranges, that flares may contribute significantly to the current ISM D content. Observational data indicate that different clouds of gas in the ISM exhibit variations in the value of D/H. We suggest that contamination of the ISM by D-enriched material ejected from stellar flares contributes to the observed D/H inhomogeneity. More precise estimates of the efficiency of D ejection from flares into the solar wind are required to evaluate this suggestion.