The reproducibility of any challenge and its sensitivity to change during therapeutic interventions should always be examined before any drug trial is conducted. Conjunctival challenge has been widely used to assess objectively the efficacy of antiallergic treatments. A double-blind, placebo-controlled, crossover study was carried out in 26 patients allergic to grass pollen to test the reproducibility of a composite symptom score during placebo and control days, and to check the ability of loratadine to increase the provocative dose inducing a positive challenge. Conjunctival challenge was carried out with increasing concentrations of a standardized orchard grass pollen extract until a composite symptom score of 5 was reached. The provocative dose inducing a positive challenge was similar for the baseline (6.94 ± 8.7 index of reactivity (IR)) and placebo days (20.0 ± 32.5 IR) and was significantly increased (P < 0.01 and P < 0.001) when patients received loratadine (117.3 ± 136.8 IR). This study shows that the composite score used was reproducible and sensitive to change, making it possible to use in drug trials.