Current information regarding the effects of coral diseases on Indo-Pacific reefs lags behind that of the Caribbean. Considering that these reefs are geographically widespread, speciose, often highly influenced by human coastal populations, and inadequately monitored, developing a baseline database is a primary management issue for local scientists. In a first attempt to quantify diseases in Micronesia, Guam reefs were assessed for disease prevalence, host abundance, and community structure. Surveys of 15 reefs revealed 6 disease states affecting 8 families of reef-building corals and highly variable prevalence between sites, ranging from 0.2 to 12.6%. Guam reefs are taxonomically diverse but dominated by the genus Porites. Coral generic host abundance showed a significant and positive link with total disease prevalence. Five out of 6 of the observed disease states affected Porites spp. (mean prevalence within the genus: 6.14 +/- 0.88%), and acroporids and pocilloporids also showed high susceptibility. As the coral genera currently most affected by diseases are those providing the most structure to Guam's reefs, disease has the potential to have significant long-term effects, highlighting an urgent need for proactive management.