To determine lymphohematopoietic malignancy incidence rates between populations living near oil-gas fields (two regions) compared with those living in areas free from oil-gas exploitation (one region) in Koprivnica-Krizevci County, Croatia, 803 new cases of lymphohematopoietic malignancies were studied for two time periods: 1971-1980 (prior to oil-gas exploitation) and 1981-2000 (during oil-gas exploitation). The results showed considerable geographic differences in the incidences of chronic myeloid leukemia and multiple myeloma. The authors conclude that proximity to oil or gas fields represents an increased population risk of developing lymphohematopoietic cancers.