The laser flare cell meter (LFCM) was used to evaluate alterations of the blood-aqueous barrier in eyes with retinal vein occlusion (RVO). In 42 eyes of 42 patients with RVO (30 branch vein occlusions, 12 central vein occlusions) aqueous flare and aqueous cells were determined and retinal fluorescein angiography was performed. Flare and cell values were compared with 59 normal age and sex-matched control eyes and correlated with fluorescein angiographic findings. Both aqueous flare (12.321 (SD 6.717) photon counts/ms) and aqueous cells (mean 3.37 cells/0.075 x 10(6)/l) were significantly increased in eyes with RVO in comparison with the normal control group (flare 4.288 (SD 1.144) photon counts/ms, cells 0.54 cells/0.075 x 10(6)/l, p < 0.0001 and p < 0.005). Flare values in eyes with central RVO (19.517 (SD 7.762) photon counts/ms) were significantly higher than in eyes with branch RVO (9.443 (SD 3.307) photon counts/ms, p < 0.0001). Significant correlations were found between flare values and area of RVO (r = 0.70, p < 0.0001) and between flare values and area of retinal non-perfusion (r = 0.76, p < 0.0001). There were also significant correlations between flare values and number of cotton wool spots (r = 0.54, p < 0.0002) and between cell counts and degree of cystoid macular oedema (r = 0.46, p < 0.006). Our findings show that aqueous protein concentration, as indicated by flare, and aqueous cells are increased in RVO and that changes of aqueous flare correlate with fluorescein angiographic findings that reflect the severity of RVO. Measurements with LFCM may be useful non-invasively to quantify changes of the ocular barrier functions in RVO.