Background Sulfasalazine (SSZ) is an anti-inflammatory and immunomodulatory drug used to treat many inflammatory diseases. Bacteria in the gut metabolize SSZ to active 5-aminosalicylic acid and inactive sulfapyridine. Sulfapyridine can crystallize in the kidney. We aimed to investigate the frequency of nephrolithiasis in patients who were diagnosed with rheumatoid arthritis (RA), ankylosing spondylitis (AS) and who received SSZ treatment retrospectively. Material and Methods We retrospectively analyzed the files of AS and RA patients in the rheumatology outpatient clinic between 2009 and 2018. We identified patients who underwent kidney ultrasonography at least six months after initiation of SSZ. One hundred six patients and 50 healthy adults were included in the study. Results Only eight patients (6 AS, 2 RA) had nephrolithiasis on ultrasonography, but none in the control group (p=0.046). In logistic regression analysis, no correlation was found between gender, age, vitamin D, parathyroid hormone, and urinary calcium excretion with SSZ use (p>0.05). Conclusion Although, it is noteworthy that these patients are prone to stone formation for various reasons. Therefore, paying attention to the patient’s hydration while using these drugs may prevent such side effects.