The invention particularly relates to crystalline (solid) forms of N-[2-[(1S)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)ethyl]-1,3-dioxo-2,3-dihydro-1H-isoindol-4-yl]acetamide), i.e. Form N, Form M, Form O and Form P. It also refers to a pharmaceutical composition or dosage form comprising said crystalline forms. The invention also concerns said pharmaceutical composition or dosage form for use in a method of treating a disease or disorder defined in the claims. Finally, the invention pertains to the use of said novel crystalline forms for the preparation of a pharmaceutical composition or dosage form.