What could a horizontal red line under my big toenail signify?

A horizontal red line under your big toenail is often a harmless finding, but it can also signal an underlying issue. One possibility is a condition called "half and half nails" (Lindsay's nails), where the nail shows a white upper half and a reddish-brown lower band. While this pattern can occasionally appear in healthy people, it's also been linked to kidney disease, liver problems, or nutritional deficiencies. Other possible causes of a red line include a splinter hemorrhage (a tiny burst blood vessel), minor trauma, or, more rarely, endocarditis or a nail-bed melanoma.

Because the causes range from completely benign to potentially serious, it's worth investigating rather than guessing. A free, instant, online symptom check can help you narrow down likely causes based on your specific symptoms and history, giving you clearer next steps—whether that's monitoring at home or booking an appointment with a doctor. It takes just a few minutes and could save you days of uncertainty.

Reviewed for medical accuracy: 07/15/2026