PURPOSE: Laser flare photometry is a new quantitative method for evaluating aqueous flare, making flare the only inflammatory parameter that can be evaluated precisely and objectively. The validity of the method already has been demonstrated in anterior segment inflammation. The aim of this study is to assess the validity and limitations of the method to quantify and monitor inflammation in uveitis with predominant involvement of the posterior segment. METHODS: Five well-defined conditions with uveitis predominant in the posterior segment were analyzed in this study: Behcet uveitis, pars planitis, posterior sarcoidosis, posterior pole toxoplasmosis, and birdshot chorioretinopathy. (1) Mean initial (pretreatment) flare was determined; (2) in the patients needing systemic steroid therapy, introduction of therapy was correlated with evoluting laser flare photometry; and (3) in patients with quiescent disease, the predictive value of a defined subclinical photometry-detected flare rise for disease recrudescence was analyzed. RESULTS: Initial pretreatment flare was 331.8 +/- 47.7 photon counts per millisecond (ph/msecond) (mean +/- standard error of the mean) for Behcet uveitis, 15.6 +/- 1.3 ph/msecond for pars planitis, 26.9 +/- 4.6 ph/msecond for posterior sarcoidosis, 7.5 +/- 1.0 ph/msecond for posterior pole toxoplasmosis, 5.8 +/- 0.7 ph/msecond for birdshot chorioretinopathy, and 4.7 +/- 0.1 ph/msecond for a group of 88 control eyes. A significant flare reduction after start of steroid therapy was seen in Behcet uveitis (78% reduction), sarcoidosis (44.8%), and pars planitis (51%), but not in toxoplasmosis or in birdshot. A small flare rise had a predictive value for disease recrudescence in 27/35 patients (predictive value, 0.77; sensitivity rate, 100%). The level of associated blood-aqueous barrier disruption for reliable follow-up of posterior uveitis was empirically determined to be 13 to 15 ph/msecond. CONCLUSION: Laser flare photometry was found to be very sensitive to monitor inflammation in uveitis of the posterior segment as long as a sufficient level of associated blood-aqueous barrier disruption (flare) was present.