Background: Human serum paraoxonase-1 (PON1) shows wide variation among different ethnic groups around the world. The aim of the present study was to determine the phenotype distribution and enzymatic activity of PON1 and ARE (arylesterase) in colorectal cancer (CRC), bladder cancer (BC) and multiple myeloma (MM) patients compared to healthy subjects. Methods: A total of 160 subjects (40 CRC patients, 40 BC patients, 40 MM patients and 40 healthy controls) were admitted to the study. The phenotype distribution of PON1 was determined by using the dual substrate (paraoxon and phenylacetate) method. Results: PON 1 and ARE activities were significantly lower in the cancer patients compared to the control group. The following phenotype distributions were assessed in the cancer and control groups: MM: 52.5% (QQ), 40% (QR), 7.5% (RR); CRC: 52.5% (QQ), 40% (QR), 7.5% (RR); BC: 55% (QQ), 35% (QR), 10% (RR); and controls: 40% (QQ), 57.5% (QR), 2.5% (RR). Conclusions: We found that MM, CRC and BC patients were associated with lower PON1, ARE and stPON1 enzyme activities compared to the healthy subjects. However, PON1 phenotypes were similar between the cancer groups and control group. Uvod: Humana serumska paraoksonaza-1 (PON-1) ispolja- va velike varijacije među različitim etničkim grupama širom sveta. Cilj ove studije bio je da se odrede fenotipska distribu- cija i enzimska aktivnost P0N1 i ARE (arilesteraze) kod paci- jenata sa kolorektalnim kancerom (KRK), kancerom bešike (KB) i multiplim mijelomom (MM) u poređenju sa zdravim ispitanicima. Metode: Studijom je obuhvaćeno ukupno 160 ispitanika (40 pacijenata sa KRK, 40 sa KB, 40 sa MM i 40 zdravih osoba). Fenotipska distribucija PON1 određena je metodom dvostrukog supstrata (paraokson i fenilacetat). Rezultati: Aktivnosti PON1 i ARE bile su značajno niže kod obolelih oa kancera u poređenju sa kontrolnom grupom. U grupama obolelih od kancera i kontrolnoj grupi utvrđene su sledeče fenotipske distribucije: MM: 52,5% (QQ), 40% (QR), 7,5% (RR); KRK: 52,5% (QQ), 40% (QR), 7,5% (RR); KB: 55% (QQ), 35% (QR), 10% (RR); i u kontrolnoj grupi: 40% (QQ), 57,5% (QR), 2,5% (RR). Zaključak: Otkrili smo vezu između pacijenata sa MM, KRK i KB i nižih enzimskih aktivnosti PON1, ARE i stPONI nego kod zdravih ispitanika. Međutim, fenotipi PON1 bili su slični u grupama sa kancerom i kontrolnoj grupi.