Ringworm of the nails or tinea unguium is the most common fungus infection of the nails, also called onychomycosis.

Onychomycosis makes the nails look white and opaque, thickened, and brittle. Those at increased risk for developing onychomycosis include:

• People with diabetes;
• People with disease of the small blood vessels (peripheral vascular disease); and
• Older women (perhaps because estrogen deficiency increases the risk of infection); and
• Women of any age who wear artificial nails (acrylic or "wraps").

Artificial nails increase the risk for onychomycosis because, when an artificial nail is applied, the nail surface is usually abraded with an emery board damaging it, emery boards can carry infection, and water can collect under the artificial nail creating a moist, warm environment favorable for fungal growth.

An alternative name is dermatophytic onychomycosis.