© Frontline Medical Communications. The prevalence of onychomycosis in the United States is estimated to be at least 12%; prevalence increases with increasing age and is highest in individuals more than 65 years of age. Trichophyton rubrum, which also causes tinea pedis, is responsible for approximately 90% of cases of toenail onychomycosis. Risk factors include a family history of onychomycosis and previous injury to the nails, as well as advanced age and compromised peripheral circulation. Patients with compromised immune function may have an increased risk for onychomycosis and are susceptible to infection with less common dermatophytes and nondermatophyte organisms.