The study was conducted to describe the temporal, geographic, demographic and treatment characteristics of Lyme disease in Delaware and to assist health planners in developing and implementing control strategies. All physician-submitted Centers for Disease Control and Prevention (CDC) follow-up Lyme disease report forms from 1989 through 1992 were reviewed for completeness. Data were gathered from completed forms only. All cases were classified according to the 1990 CDC surveillance case definition. Cases were further subdivided into two groups. Antibiotic usage patterns were then identified for each group. Data on the percentage of infected ticks by county were obtained from a 1988 study conducted by the University of Delaware; Delaware Health and Social Services, Division of Public Health; and the Department of Natural Resources and Environmental Control. Reported cases of Lyme disease increased 246 percent between 1989 and 1992. The 1992 statewide incidence rate was 12.6 cases per 100,000 population. Whites were four times more likely to contract Lyme disease than were blacks. The majority of cases were reported between June and October. The number of patients being treated with oral antibiotics for localized disease for three weeks or longer increased from 52 percent in 1991 to 94 percent in 1992. Ixodid ticks infected with Borrelia burgdorferi were found in all three counties. The Delaware State Board of Health made Lyme disease reportable in September 1989. This requirement increased the quality of Lyme disease surveillance; however, the disease is probably under-reported since Delaware does not actively solicit Lyme disease reports. Delaware's case data reflect national data which indicate an increase in reported cases. A trend toward longer duration of treatment for localized Lyme disease is evident.