Nephrolithiasis (Kidney Stones): This condition is a common cause of bilateral flank pain, especially if the stones are moving or obstructing the ureters. The pain is often severe and can radiate to the groin.

Pyelonephritis: An infection of the kidneys that can cause bilateral flank pain, fever, and other urinary symptoms. It's more common in women and individuals with urinary tract abnormalities.
Urinary Tract Infection (UTI): While often causing lower urinary tract symptoms, severe or upper UTIs can lead to bilateral flank pain.
Musculoskeletal Strain: Overuse or strain of the back muscles can cause bilateral flank pain, especially in individuals with a history of heavy lifting or bending.

Abdominal Aortic Aneurysm: Although it typically presents with abdominal pain, a large or leaking aneurysm can cause bilateral flank pain. This is a medical emergency.
Sickle Cell Crisis: Individuals with sickle cell disease can experience severe pain episodes, including bilateral flank pain, due to vaso-occlusive crises affecting the kidneys.
Pulmonary Embolism: In rare cases, a large pulmonary embolism can cause referred pain to the flanks, especially if there's associated pleuritic chest pain.

Polycystic Kidney Disease (PKD): A genetic disorder leading to the growth of numerous cysts filled with fluid in the kidneys, which can cause bilateral flank pain due to cyst rupture or hemorrhage.
Renal Cell Carcinoma: Although typically asymptomatic in early stages, advanced renal cell carcinoma can cause bilateral flank pain, especially if there's metastasis or invasion into surrounding structures.
Amyloidosis: A condition characterized by the deposition of amyloid proteins in various organs, including the kidneys, which can lead to bilateral flank pain among other systemic symptoms.