Methods of treating, managing or preventing psoriasis or psoriatic arthritis are disclosed. Specific methods encompass the administration of cyclopropyl-N-{2-[(1S)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)eth- yl]-3-oxoisoindoline-4-yl}carboxamide, alone or in combination with a second active agent. Pharmaceutical compositions and single unit dosage forms are also disclosed.