A patient was admitted to the medical emergency department by his family physician. His complaints were weakness and fatigue for more than one week. Four days before admission, he went to his general practitioner for these complaints and also for painful elbows. His physician prescribed diclofenac and esomeprazole. Upon presentation, he had high systolic/diastolic blood pressure (>180/>90 mm Hg, measured repeatedly), and otherwise normal parameters. He had gained 6.5 kg in body weight. Clinical examination was normal, except for very mild bilateral malleolar edema. Routine blood tests showed a strongly elevated serum creatinine, hyperkalemia, and elevated lactate dehydrogenase. Haptoglobin levels were normal. Urinalysis showed a normal sediment, urine and blood cultures remained sterile. Ophthalmoscopy was completely normal, as was a routine chest X-ray. Renal ultrasound demonstrated kidneys with a diameter of 13 cm. Due to uncontrollable hypertension, our patient was hospitalized at the intensive care department where intravenous nifedipine was started, with good instantaneous control of blood pressure. Because of increasing potassium levels acute hemodialysis was started within 24 h after admission. Differential diagnosis consisted of diclofenac- or esomeprazole-induced interstitial nephritis or rapidly progressive glomerulonephritis. A renal biopsy was performed within 72 h after admission. The kidney biopsy showed an overwhelming inflammatory cell infiltrate consisting of a monoclonal lymphocytic cell population. However, the numerous mitotic figures, polyploidy, and prominent nucleoli present, were indicative of a lymphoma. Additional stainings confirmed a non-Hodgkin diffuse large-cell B-cell lymphoma. Treatment with R-CHOP (rituximab, cyclophosphamide, doxorubicine, vincristine, and prednisolone) was initiated with very good clinical and biochemical response, yet only mild recovery of kidney function. Occasionally the kidney is involved as an extranodal non-Hodgkin lymphoma (NHL) localization. However, a primary presentation of acute kidney failure due to lymphoma localization is extremely rare. Our case demonstrates that early renal biopsy is indispensable for fast and adequate diagnosis and treatment.