BACKGROUND: Marfan syndrome (MFS) is a genetic disorder that affects multiple organ systems, including the eye. The most common ocular manifestations include ectopia lentis and retinal detachment. The current literature qualitatively cites that MFS patients have miotic or "poorly dilating" pupils. This study was the first to quantitatively assess pupillary function in MFS patients. MATERIALS AND METHODS: 57 eyes from 29 MFS patients, 36 eyes from 18 pediatric age- and gender-matched controls, and 44 eyes from 22 adult age-matched controls were measured in a clinic-based cross sectional study. Pupillometry data were measured in scotopic conditions using the handheld NeurOptics PLR-200 Pupillometer (NeurOptics, Irvine, CA, USA). Data obtained with the pupillometer were maximum and minimum diameter, constriction percentage, latency, average and maximum constriction velocities, average dilation velocity, and 75% recovery time (T75). RESULTS: Pediatric patients with MFS had significantly slower average constriction velocity measurements (beta = 0.65, p = 0.0003), maximum constriction velocity measurements (beta = 0.51, p = 0.0150) and average dilation velocity measurements (beta = -0.19, p = 0.0029) compared to control patients. In the adult cohort, results indicated significantly slower average dilation velocity measurements (beta = -0.13, p = 0.0077) compared to controls. CONCLUSIONS: Our data highlight pupillary parameters within a population of MFS patients under scotopic conditions. Constriction and dilation velocities were slower in the pediatric MFS patients compared to age- and gender-matched controls, and dilation velocities were slower in the adult MFS patients compared to age-matched controls. These findings, for the first time, quantitatively demonstrated differences in pupillary function in patients with MFS.