Mononeuria patula (Michx.) Dillenb. and Kadereit (formally known as Minuartia patula; Caryophyllaceae) is a winter annual found in barrens, glades, rocky outcrops, and dolomite prairies of the eastern and central United States. This species is rare in Illinois, Indiana, and Ohio, and vulnerable in Kentucky, Missouri, and Virginia. In a two-year study we examined several aspects of the reproductive ecology of M. patula at five populations in northeastern Illinois. We used a combination of field-based studies (i.e., fruit set and seed set, breeding system, population size estimation), field observations (i.e., phenology), laboratory studies (i.e., seed germination, pollen:ovule ratio, floral morphometrics) and historical data (i.e., population size and weather data) to answer several questions: (1) What can data on the reproductive ecology of M. patula tell us about the species?; (2) Are there differences in reproductive output among populations of M. patula?; and (3) Can variation in the population size of M. patula be related to reproductive output and/or local weather patterns? Based on caging studies, flower phenology, and pollen to ovule ratio, M. patula can be considered a facultative autogamous species. Also, although significant differences were found among populations of M. patula for fruit set, seed set, and seed germination, fruit set was relatively high (above 60%), but seed set and seed germination were relatively low (less than 50%) regardless of the population. No correlation was found between population size and reproductive output or local weather data. The results of this study can be used to develop a conservation strategy for M. patula across its range in the U.S.