Mesalazine (5-aminosalicylic acid, 5-ASA),is an anti-inflammatory drug well-established as first-line treatment in the management of inflammatory bowel disease. Nephritic colic has been described as an uncommonside effect, but very few mesalazine nephrolithiasis have been published in the literature, probably due to its underdiagnosis for not using appropriate methodology for calculi analysis. We present two cases of lithiasis in patients treated with mesalazine at high dosage, an adverse effect to betaken into account as a drug lithiasis and which is not even mentioned in the patient information leaflet.