Whole blood viscosity (WBV) and hematometric indices of erythrocytes as red blood cell count (RBC), mean erythrocyte volume (MCV), hemoglobin (HGB), hematocrit (HCT), mean hemoglobin content of erythrocytes (MCH), HGB/HCT values (MCHC) and red blood cell distribution width (RDW) have been studied in a group of 15 chronic opioid addicts under methadone maintenance therapy with mean age 26.53 +/- 7.34 years. WBV elevation and changes in MCV, HGB, HCT, RDW were found in intravenous drug users compared to healthy individuals. As well, RBC was decreased leading to an increase in MCH and MCHC values. Correlation analysis suggested that the correlation among the RBC, HGB, HCT and WBV was the closest. Heroin macrocytosis (heroin macrocytic anemia) was established, related with the increased RDW in chronic heroin abusers. The results are in accordance with data revealing abnormal effects of alcohol and other drugs on whole blood rheology and hematometric/morphometric characteristics of erythrocytes.