A very rare case of acetylsulfapyridine nephrolithiasis is presented in a 54-year-old female patient who had been prescribed sulfasalazine (6 x 500 mg/day) because of psoriatic arthritis for the last 9 years. The patient's renal function was only slightly impaired. Reflectance infrared spectroscopy and gas chromatography-mass spectrometry allowed the identification of the chemical nature of the stone. As acetylsulfapyridine is a metabolite of sulfasalazine, administration of the latter drug was the cause of the nephrolithiasis.