The incidence rate per 100,000 population by gender and age group was calculated for the 24 Maryland jurisdictions using 1,392 cases reported to the Lyme Disease Registry during 1993 to 1996. The overall incidence rate for the state was 7.0 cases per 100,000 inhabitants. Males (incidence rate = 7.7) were 1.2 times more affected than females (incidence rate = 6.2) (p < 0.001). The bimodal overall age distribution shows a peak in the 10 to 19 age category (incidence rate = 8.5) and in the 50 to 59 age group (incidence rate = 8.9), and in the 20 to 29 age group (incidence rate = 3.9). The incidence rate by jurisdiction ranged from 0.7 in Allegany County from Western Maryland to 88.3 in Kent County on the Upper Eastern Shore. Results from this study show that counties from the Upper Eastern Shore region had the highest specific incidence rate by age group/gender/jurisdiction while counties from Western Maryland had the lowest specific incidence rates and have not reported Lyme disease patients younger than 10 or older than 59 years of age. This difference might be explained primarily by the kind of outdoor activities in which the different age groups are involved and the relative abundance of Ixodes scapularis infected with the etiologic agent of Lyme disease, Borrelia burgdorferi, in these two regions.