PROBLEM/CONDITION: Lyme disease is a multisystem disease that occurs in North America, Europe, and Asia. In the United States, the etiologic agent is Borrelia burgdorferi sensu stricto, a spirochete transmitted to humans by infected Ixodes scapularis and I. pacificus ticks. The majority of patients with Lyme disease develop a characteristic rash, erythema migrans (EM), accompanied by symptoms of fever, malaise, fatigue, headache, myalgia, or arthralgia. Other manifestations of infection can include arthritis, carditis, and neurologic deficits. Lyme disease can be treated successfully with standard antibiotic regimens. REPORTING PERIOD: 1992--2006. DESCRIPTION OF SYSTEM: U.S. health departments report cases of Lyme disease voluntarily to CDC as part of the National Notifiable Disease Surveillance System. Variables collected include patient age, sex, race, county and state of residence, date of illness onset, and reported signs and symptoms. RESULTS: During 1992--2006, a total of 248,074 cases of Lyme disease were reported to CDC by health departments in the 50 states, the District of Columbia, and U.S. territories; the annual count increased 101%, from 9,908 cases in 1992 to 19,931 cases in 2006. During this 15-year period, 93% of cases were reported from 10 states (Connecticut, Delaware, Massachusetts, Maryland, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island, and Wisconsin). Incidence was highest among children aged 5--14 years, and 53% of all reported cases occurred among males. More than 65% of patients with EM had illness onset in June and July, compared with 37% of patients with arthritis. INTERPRETATION: Lyme disease is the most commonly reported vectorborne illness in the United States. The geographic distribution of cases is highly focused, with the majority of reported cases occurring in the northeastern and north-central states. During 1992--2006, the number of reported cases more than doubled. A disproportionate increasing trend was observed in children and in young males compared with other demographic groups. PUBLIC HEALTH ACTION: The results presented in this report underscore the continued emergence of Lyme disease and the need for tick avoidance and early treatment interventions. Public health practitioners can use the data presented in this report to target prevention campaigns to populations with increasing incidence (i.e., children and young males).