Multiple primary malignancies, metachronous or synchronous, in a single patient are relatively rare event with the increase of incidence in recent decades. The aim of this research is to study their incidence in patients hospitalized at the Division of Radiotherapy, Department of Oncology, University of Zagreb, School of medicine, University Hospital Centre Zagreb from 2003 to 2009. The incidence of multiple primary malignancies was 2.4%. Among 103 patients, 97 had two, and 6 three primary tumors. Eighty-three cases were metachronous, while 20 cases were synchronous malignancies. The frequency was higher in females than males and their age at diagnosis of tumors was younger than in males. The most common tumor combinations in males were: prostate cancer-digestive system malignancy (especially colorectal cancer) and viceversa, and hematological malignant tumors-digestive system malignancy; while in women there were: breast cancer-cancer of contralateral breast and hematological malignant tumors (especially lymphoma non Hodgkin)-breast cancer. The incidence of multiple primary malignancies is expected to increase due to the better screening programs for early detection of malignancies as well as considerable improvement in their treatment and longer life expectancy.