Red yeast rice, traditionally used in Asian cuisine and increasingly marketed as a dietary supplement for cholesterol management, has recently been linked to kidney dysfunction in Japan. In late 2023 to early 2024, multiple cases involving specific Beni-koji (red yeast rice) tablets from three different Beni-koji preparations, prompted a safety reevaluation. Although citrinin, a known nephrotoxin of red yeast rice, was not produced by the implicated strains, new safety concerns emerged. Here, we aimed to investigate the clinical, laboratory, and pathological features of affected patients with a two-phase nationwide survey of Japanese nephrologists. The initial survey captured clinical presentations, while the follow-up survey tracked changes in kidney function and gathered pathological data. Statistical analyses included trend assessments across estimated glomerular filtration rate (eGFR) categories and mixed-effects models for eGFR trajectories. Of 192 patients, 94.1% presented with low eGFR (under 60 ml/min/1.73m