Eighty-six patients with retinal vein occlusion (37 with central, 49 with branch vein occlusion) and 31 patients with treated essential arterial hypertension were investigated for comparison to an age-matched control group. Serum C-reactive protein (CRP) levels, erythrocyte sedimentation rate (ESR) and plasma viscosity were measured. Serum CRP levels (log10 values) were significantly elevated in patients, with hypertension (P less than 0.001) or with retinal vein occlusion (P less than 0.001) compared to control. The highest mean value of serum CRP were found in patients with both hypertension and retinal vein occlusion, and the values of ESR and serum CRP were significantly higher in this group when compared to normotensive patients with retinal vein occlusion (P less than 0.05 and P less than 0.01, respectively) or to control (P less than 0.001). Increased inflammatory activity may be present in patients with hypertension or retinal vein occlusion. In particular hypertensive patients with elevated serum CRP levels may be more at risk of developing retinal vein occlusion.