The best known symptom of early disease is the erythema migrans or EM rash, which can occur at the site of the tick bite. Sometimes multiple rashes are present. Untreated EM rashes expand and clear over days to weeks though some have been known to last over a year. EM rashes are usually solid coloured, ranging from faint pink to deep red. Less than 20% of all EM rashes have the classic "bull's-eye" appearance.
Erythema migrans (Source: DermNetNZ.org)
It is often said that the rash should be over 5cm across, but a smaller rashes have been documented.
It is unclear how many people with Lyme disease develop an identifiable EM rash. In the USA, CDC surveillance case data documents that about 30% of confirmed cases lack a reported rash. An EM rash is diagnostic of Lyme disease, meaning that people who have the rash have Lyme disease. Because blood tests may be falsely negative in early Lyme Disease, testing is not recommended. Antibiotic treatment should begin as soon as an EM rash is identified.