Brassinosteroids (BRs) are a relatively recently discovered group of phytohormones that are essential for normal plant growth and development. They participate in regulation of numerous vital physiological processes in plants, such as elongation, germination, photomorphogenesis, immunity and reproductive organ development. Structurally they are very similar to animal steroid hormones and include about 70 polyhydroxylated sterol derivatives. They are found at low levels in practically all plant organs. Recent studies have indicated that BRs have antiproliferative, anticancer, antiangiogenic, antiviral and antibacterial properties in animal cell systems, and thus have potential medical applications. Among others, BRs can inhibit replication of viruses in confluent human cell cultures, sometimes with high selectivity indexes, inducing cytotoxic effects in various types of cancer cells but not normal human cells. Thus, they include promising leads for developing potent new anticancer drugs. The aims of this article are to overview chemical characteristics, biological activities and the potential medical applications of natural BRs.