Objective: Oporto's Hospital Centre is one of the portuguese hospitals with more renal transplantation activity (performed since 1982). Although increasingly rare, opportunistic infections (OI) in transplanted patients remain a major diagnostic challenge and are associated with high mortality rate. Method: In order to evaluate the incidence of OI in renal transplant patients and to identify the location of infection, the respective techniques of diagnosis used and the survival time after infection, we conducted a retrospective study using the Nephrology Department's database on renal transplants. We consulted the registries from 2004 to 2012. Results: We investigated 2041 cases and found 82 cases of OI caused by Herpes virus (n=7), Cytomegalovirus (n=27), Polyomavirus (n=16), Aspergillosis (n=4), Alternaria (n= 2), Mucormycosis (n=1), Candidiasis (n=12), Tuberculosis (n=9), Cryptococcus (n=2) and Pneumocystis (n=2). The lung and urogenital system were the most affected systems. Kidneys were affected in 3 cases respectively by mucormycosis, tuberculosis and cryptococcus. Of all the cases of OI, 39 were diagnosed by the Department of Anatomic Pathology (21 by biopsy; 16 by cytology; 2 by biopsy and cytology). Conclusion: In many cases the diagnosis could only be performed through histologic/cytologic examination. Prompt diagnosis and treatment are necessary to avoid life threatening complications and may greatly improve prognosis.