AbstractOn the basis of the idealized structure of an atomic medium described by an assembly of independent oscillators, expressions for the stored electric and magnetic energy densities in absorptive and dispersive medium are derived. Such formulas are also valid for a left‐handed medium (LHM), where the permittivity and permeability are simultaneously less than zero within a frequency band. These formulas are completely equivalent to those derived from a general definition of electromagnetic energy density using the classical electromagnetics. It has been shown that the stored energy densities, no matter in a right‐handed medium or in a left‐handed medium, are always positive. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2288–2291, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21899