Many fungi in the Basidiomycota have a dimorphic life cycle, where a monokaryotic yeast form alternates with a dikaryotic hyphal form. Most of the dimorphic basidiomycetes are pathogenic on plants, animals or other fungi. In these species, infection of a host appears to be closely linked to both dimorphism and the process of sexual reproduction. Sex in fungi is governed by a specialized region of the genome known as the mating type locus that confers cell-type identity and regulates progression through the sexual cycle. Here we investigate sexual reproduction and lifestyle in emerging human pathogenic yeasts and plant pathogenic smuts of the Basidiomycota and examine the relationship among sex, dimorphism and pathogenesis.