Four crystal modifications of acyclovir were isolated by recrystallization and characterized by powder X-ray diffractometry, differential scanning calorimetry, and thermogravimetric analysis. It was confirmed that Form 3 is a hydrate and Form 4 is an acetic acid solvate. The dissolution patterns of three crystal forms of acyclovir were studied in water at $37{pm}0.5^{circ}C$, 90 rpm for 120 minutes. The amount dissolved at 120 minutes was highest for Form 1, followed by Form 2 and Form 3. After storage of 25 hours at 0% RH (silica gel, $20^{circ}C$) Form 3 was transformed to Form 2.