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There is a great need for valid outcome measures of functional improvement for impending clinical trials of targeted interventions for fragile X syndrome (FXS). Families often report conversational language improvement during clinical treatment, but no validated measures exist to quantify this outcome. This small-scale study sought to determine the feasibility, reproducibility, and clinical validity of highly structured expressive language sampling as an outcome measure reflecting language ability.Narrative and conversation tasks were administered to 36 verbal participants (25 males and 11 females) with FXS (aged 5–36 years, mean, 18 ± 7 years). Alternate versions were used with randomized task order at 2- to 3-week intervals (mean, 19.6 ± 6.4 days). Audio recordings of sessions were transcribed and analyzed. Dependent measures reflected talkativeness (total number of utterances), utterance planning (proportion of communication [C] units with mazes), articulation (proportion of unintelligible/partly unintelligible C-units), vocabulary (number of different word roots), and syntactic ability (mean length of utterance [MLU] in words). Reproducibility of measures was evaluated with intraclass correlation coefficients (ICC).All participants could complete the tasks. Coded data were highly reproducible with Pearson's correlations at p < .01 for all measures and ICC values of .911 to .966 (conversation) and .728 to .940 (narration). Some measures including MLU and different word roots were correlated with expressive language subscale scores from the Vineland Adaptive Behavior Scale.These expressive language sampling tasks appear to be feasible, reproducible, and clinically valid and should be further validated in a larger cohort, as a promising means of assessing functional expressive language outcomes during clinical trials in FXS.