Background <p>The purpose of this study was to clarify the characteristics of newly diagnosed systemic lupus erythematosus (SLE) patients with or without kidney involvement in Japan.</p> Methods <p>We used electronic data of SLE patients in the National Database of Designated Intractable Diseases of Japan who newly registered between 2015 and 2017. We analyzed patients within one year of disease onset. Kidney involvement was defined as any of the following: urinary protein ≥ 0.5&#xa0;g/day, granular casts, a clinical diagnosis of nephrotic syndrome, acute or chronic renal failure, or rapidly progressive glomerulonephritis, an estimated glomerular filtration rate (eGFR) &lt; 60&#xa0;mL/min/1.73 m², lupus nephritis confirmed by renal biopsy, or hemodialysis.</p> Results <p>Among 2315 SLE patients, 1088 (47.0%) had kidney involvement. Patients with kidney involvement more frequently exhibited symptoms such as pulmonary hemorrhage, pulmonary infarction and disturbance of consciousness, pericarditis, and hemolytic anemia whereas manifestations such as arthritis, aseptic meningitis, discoid rash, photosensitivity, and Raynaud’s phenomenon were less common compared with those without kidney involvement. Anti-DNA antibody positivity was higher and complement levels (C3, C4, and CH50) were lower in patients with kidney involvement. In addition, concomitant glucocorticoid pulse therapy and immunosuppressive drugs were more frequently used in patients with kidney involvement.</p> Conclusion <p>In this nationwide cohort, nearly half of newly diagnosed Japanese patients with SLE had kidney involvement and showed a distinct pattern of systemic manifestations, autoantibody profiles, and treatment intensity. These findings provide important insights into the epidemiology and pathophysiology of kidney involvement in SLE in Japan.</p>