Most people who get Lyme disease develop a red rash called erythema migrans (EM) within a month after the tick bite. The average length of time before the rash appears is 7 to 14 days, but symptoms can develop anywhere from 3 to 31 days after being bitten.

• The rash usually starts as a small red area and then expands over the course of several days. It may grow to be 1 ft (0.3 m) across. Sometimes the rash clears in the center, so that it looks like a bull's-eye.
• The rash may be warm to the touch, but it is usually not painful or itchy.
• Common sites for the rash include the thighs, groin, trunk, armpits, waistline, and behind the knees. The rash is easy to miss if it is in an area you can't see, such as the scalp or buttocks, which are common rash sites in children. You also are more likely to miss ticks that have attached in these areas.
• Patches of red rash may develop elsewhere as the Lyme disease bacteria circulate through the blood and come to rest in other areas of the skin.
• The rash usually fades within 3 to 4 weeks even if you are not treated for Lyme disease.