Fungus-growing ants (Myrmicinae: Attini) live in an obligate symbiotic relationship with a fungus that they rear for food, but they can also use the fungal mycelium to cover their brood. We surveyed colonies from 20 species of fungus-growing ants and show that brood-covering behavior occurs in most species, but to varying degrees, and appears to have evolved shortly after the origin of fungus farming, but was partly or entirely abandoned in some genera. To understand the evolution of the trait we used quantitative phylogenetic analyses to test whether brood-covering behavior covaries among attine ant clades and with two hygienic traits that reduce risk of disease: mycelial brood cover did not correlate with mutualistic bacteria that the ants culture on their cuticles for their antibiotics, but there was a negative relationship between metapleural gland grooming and mycelial cover. A broader comparative survey showed that the pupae of many ant species have protective cocoons but that those in the subfamily Myrmicinae do not. We therefore evaluated the previously proposed hypothesis that mycelial covering of attine ant brood evolved to provide cocoon-like protection for the brood.