We present a case of a man in his late 50s with a history of metastatic prostate carcinoma requiring bilateral ureteric stenting. He was admitted with increasing confusion and lethargy. He was diagnosed with sepsis and an acute kidney injury (AKI). Clinical suspicions of an obstructive component to his AKI were not confirmed by an ultrasound scan, which showed a unilateral hydronephrosis unchanged from a scan 1 month previously. A nephrostomy was performed, and frank pus aspirated. The patient's clinical state improved steadily thereafter. Patients who are dehydrated, or who have suffered from malignant or fibrotic processes affecting the retroperitoneum, may present with urinary obstruction without a corresponding increase in urinary tract dilation. Additionally, there must be a suspicion of pyonephrosis in a symptomatic patient with known hydronephrosis. Clinicians should be aware that clinical suspicions of urinary obstruction not demonstrated on ultrasound scanning require further investigation.