Human actinomycosis is an infrequent chronic infection caused by gram-positive anaerobic bacteria with predominantly cervicofacial and intestinal manifestation. Retroperitoneal abscess formation displays a very rare localisation and is mostly incidentally diagnosed by histological examination. We report on a 44-year-old woman with left-sided flank pain and retroperitoneal abscess formation diagnosed by CT scan. Case history revealed preceding nephroureterectomy of the left kidney due to loss of kidney function and recurrent ureteral-vaginal fistulas. After CT scan-guided puncture and negative bacterial culture, actinomycosis could only be diagnosed by histopathological examination. Subsequently, besides abscess drainage calculated antibiotic therapeutic regimen was initiated. During the follow-up of 9 months there was no local or systemic recurrence. In the present case report, aetiology, clinical symptoms as well as diagnostic and therapeutic consequences are discussed.