AbstractPolyploidy, which is common in plants, can confound taxon recognition and hence conservation assessments. In the taxonomically complex genusRhododendron, 25 % of the over 1,300 taxa are considered under threat and 27 % Near Threatened or Data Deficient, with their taxonomy needing to be resolved urgently. Although ploidy levels ofRhododendrontaxa range from diploid (2x) to dodecaploid (12x) according to previous reports, the extent of polyploidy across the genus has not been examined. We first summarized the taxonomic distribution of polyploids in the genus based on the literature. Then as a case study, we estimated ploidy levels of 47 taxa in subsectionMaddenia(subgenusRhododendron, sectionRhododendron) using flow cytometry, together with verification of meiotic chromosome counts for representative taxa. The summary of reported ploidy inRhododendronindicates that polyploidy is most common in subgeneraPentantheraandRhododendron. In subsectionMaddenia, all examined taxa are diploids except for theR. maddeniicomplex that shows a high ploidy variation (2–8x, 12x). We investigated ploidy level of 12 taxa in subsectionMaddeniafor the first time, and estimated genome sizes of twoRhododendronspecies. Knowledge of ploidy levels will inform phylogenetic analysis of unresolved species complexes. Overall, our study of subsectionMaddeniaprovides a model for examining multiple issues including taxonomic complexity, ploidy variation and geographic distribution in relation to biodiversity conservation.