An 18-year-old long-term Norwegian resident of Somali origin was submitted to hospital with bloody diarrhoea, fever, weight loss and abdominal pain. On initial colonoscopy, colitis with segmental appearance was seen. Apart from a single polymerase chain reaction (PCR) from gastric aspirate staining, PCR and culture for acid-fast bacilli revealed negative results from the multiple samples taken including sputum, gastric fluid, stool, urine and intestinal mucosa. On physical examination and CT scan, there was no evidence of ascites, lymph node enlargement or pathologic pulmonary findings. Although the diagnosis was uncertain, tuberculostatic therapy was initiated. As the conformational testing of the PCR and the microbiological work-up remained negative and the patient's condition did not improve, tuberculostatic treatment was stopped and Crohn's disease was stated as the most likely diagnosis. Although the patient improved clinically under therapy with prednisolone, newly appearing fistulas deriving from the ascending colon were noted on follow-up. Thus tuberculostatic treatment was restarted. However, signs of an acute abdomen appeared and laparotomy was performed, thereby revealing a peritoneal spread of nodules. Resection of the ileum and ascending colon was performed. Diagnosis of intestinal tuberculosis with peritoneal spread was made by histology from resected bowel specimens showing caseating granulomas and a positive PCR result. The patient's condition improved after resection of the highly inflamed bowel segments and tuberculostatic therapy. Our case report shows the difficulty of proving intestinal tuberculosis by microbiological testing, macroscopic features on colonoscopy, histology, imaging such as CT scan and by empirical therapy. Therefore, in cases of colonic inflammation, where intestinal tuberculosis is an important differential diagnosis, a more aggressive diagnostic approach such as explorative laparoscopy should be considered.