We describe a 14-year-old female who presented with persistently elevated nighttime urinary protein excretion without additional clinical symptoms. She had no evidence of intrinsic renal disease on physical examination or laboratory studies. Ultrasound examination of the abdomen revealed a large cyst arising from the spleen. CT scan showed compression of the left renal vein by the splenic cyst. Removal of the cyst resulted in resolution of her proteinuria. Entrapment of the left renal vein (nutcracker syndrome) remains a rare but important cause of elevated protein excretion.