Stereomerically pure (+)-2-[1-(3-Ethoxy-4-methoxyphenyl)-2-methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione, substantially free of its (−) isomer, and prodrugs, metabolites, polymorphs, salts, solvates, hydrates, and clathrates thereof are discussed. Also discussed are methods of using and pharmaceutical compositions comprising the (+) enantiomer of 2-[1-(3-Ethoxy-4-methoxyphenyl)-2-methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione are disclosed. The methods include methods of treating and/or preventing disorders ameliorated by the reduction of levels of TNF-α or the inhibition of PDE4.