Corneal allograft rejections is a major complication of penetrating keratoplasty (PK). We used the laser flare-cell meter that allows, for the first time, non-invasive quantification of aqueous flare in vivo to analyze alterations of the blood-aqueous barrier following uncomplicated PK and in acute corneal graft rejection. Applications of the laser flare-cell meter appears promising for following up patients after PK and for detecting early corneal allograft rejection.