In this study, the relationship was explored between flowering potential and plant size in a population of Syneilesis palmata (THUNB.) MAX., Usannamul. For this perennial shade-tolerant herb, the flowering rate, plant size, and changes that occurred in the next year after flowering were monitored in two different types of natural stands: moderately shaded (MS; for 11 years) and severely shaded (SS; 7 years). The correlation coefficients (CC) between inflorescence number and other factors, i.e., widths of the first leaf, second leaf, or total leaf breadth, were significant at the 1% level in both stands. In particular, the CC was >0.9 between the number of inflorescences and total leaf breadth. Although the mean flowering rates averaged 6.8% in M5 and 8.6% in SS, those values varied widely, ranging from 0 to 53% among years. These rates were not related to petiole length but did increase in proportion to the width of the leaf blade. Flowering began when plants moved beyond the medium size class; the rate in the MS environment was 100% for the largest-sized plants. For plants in the SS, however, the highest rate was 35%. Over the 11 years, 62.8% of all plants in the MS never flowered, while another 30.8% bloomed only once. In comparison, plants observed for 7 years in the SS showed a non-flowering rate of 78.1% and a one-time-only rate of 19.2%. In the next year after flowering, 57.6% of the MS plants exhibited reduced growth while 25.8% of those individuals each split into two plants. In the SS, 37.8% had decreased development the following year, and a mortality rate of 37.8%.