As a result of animal welfare issues, the use of furnished cages has been proposed. Blood viscosity and osmotic stability can be used as an indication of oxidative stress. There has, however, not been any study investigating the blood viscosity and erythrocyte osmotic stability of laying hens reared in battery and furnished cages, hence the reason for this experiment. 790 laying hens were reared in large and small furnished cages and battery cages between 14 and 60 weeks. The whole blood viscosity and erythrocyte osmotic stability were determined at the end of the experiment. The study showed that there was no significant difference in the whole blood viscosity of laying hens reared in the three cage types, while hens reared in battery cages had the highest number of stable red blood cells. This could be an indication that the stress level in the three cages is similar, while birds are more osmotically stable in the battery cages.