BACKGROUND Microscopic tumor foci have been detected incidentally on renal biopsy, including renal cell carcinoma and renomedullary interstitial cell tumor (medullary fibroma). A report is presented of a case of an incidental finding of microscopic renal angiomyolipoma that was diagnosed and completely excised on core needle biopsy. CASE REPORT A 44-year-old woman was referred to our hospital for evaluation of persistent mild proteinuria. Three years previously, she was diagnosed with Cushing's syndrome associated with a right adrenal cortical adenoma, which was successfully treated with unilateral adrenalectomy. At the time of surgery, abdominal computed tomography (CT) showed no renal lesions. During the present admission, a renal biopsy was performed that showed minimal changes in the renal glomeruli and interstitium. Immunofluorescence showed weakly positive staining for IgM in the glomeruli and no dense deposits. A microscopic focus of a predominantly spindle-cell tumor was found in the corticomedullary region. Immunohistochemistry showed positive immunostaining for HMB-45, Melan-A, and alpha-smooth muscle actin (ASMA), which supported a diagnosis of angiomyolipoma. Abdominal ultrasound at one-year follow-up showed no evidence of residual renal tumor. CONCLUSIONS To our knowledge, this is the first reported case of a completely excised incidental microscopic renal angiomyolipoma. This case demonstrated that even when imaging findings are normal, renal biopsy may detect microscopic foci of primary renal tumors.