Santuario de la Naturaleza Yerba Loca (SN Yerba Loca), Metropolitan Region (MR), 33º S, Chile is analyzed for its conservation value and efficiency in protecting native vascular plants in a regional context. The reserve's flora of 500 species and subtaxa was evaluated for species richness, endemism, range size and marginally distributed taxa, using species-area analysis, and tendencies in the floras of the MR (1.434 species and subtaxa) and MR-Fifth regions (1,841 species and subtaxa) to set the regional pattern. The reserve (0.7 % of MR land area and 0.3 % MR-Fifth land area) contains 34 % of the MR and 27% of the MR-Fifth floras, and around 16-17 % of the mediterranean-climate area (regions IV-VIII) flora of central Chile. Veech's Relative Richness Index (RRI) revealed that SN Yerba Loca houses exaggerated richness in relation to its land area (28 % more species than expected from the regional model). However, endemism rates (35 % Continental Chile endemics, 22 % Mediterranean endemics, 3% MR-Vth endemics) are statistically lower than in the MR (44 %, 29 %, 9 %) and the MR-Vth (48 %, 31 %, 11 %) floras, and SN Yerba Loca houses proportionately fewer MR endemics (2 %) than the MR (6 %). Compared with the regional floras, the reserve contains statistically fewer marginally distributed species, and range size (median = five administrative regions) is significantly larger. The reserve's outstanding species richness compensates for its low endemism rates bringing the absolute number of endemics to 92 % of the regional expectation. Corresponding values for marginally distributed species are 81 % (northern limits), 63% (southern limits) and for median and shorter range taxa, 100 %. It is concluded that SN Yerba Loca is a highly efficient reserve from the point of view of vascular plant conservation, and represents an excellent conservation choice. SN Yerba Loca and MN El Morado (a second state protected area in the MR), conservatively, house 39 % of the native vascular plant flora of the MR (30 % of that of the MR-Fifth) on 0.9 % of MR land area. Our study emphasizes that relatively small land areas in central Chile can house significant amounts of biodiversity, and that moreover, the RM and RM-Fifth are areas of high species richness within the central Chilean biodiversity hotspot. The outstanding conservation value of SN Yerba Loca calls for a management plan designed to assure the integrity of the reserve under increasing pressures from the Metropolitan Region with six million inhabitants