Nearly 40 years have passed since the publication of the first report showing higher cancer risks in recipients of organ transplants. Thereafter, studies carried out in immunosuppressed persons have greatly expanded our knowledge on the spectrum of cancers associated with infections. Clinical investigations following the expanding practice of organ transplantation, and since the 1980s, studies on human immunodeficiency virus (HIV)-infected persons have thereafter confirmed and extended these early observations. The comparison of the spectrum of cancers seen in excess in these two groups of populations offers an original viewpoint into the association of immunosuppression and cancer. Combining longitudinal data from different cohorts of HIV-infected persons and of transplant recipients represents a further tool for better quantifying the spectrum of cancers associated with immunodepression. In this chapter, the use of this methodologic approach in southern Europe is illustrated and limitations are discussed.