The relative abundance of, and spirochetal-infection rates in, adult ixodid ticks from eight Lyme borreliosis clinical-case areas and two comparison areas were investigated in northern California from late fall to early spring, 1984-87. The western black-legged tick (Ixodes pacificus) was the most abundant species at seven of nine sites yielding specimens as determined with a tick drag method. The Pacific Coast tick (Dermacentor occidentalis) was the most abundant species at two sites, and lesser numbers of this tick and the American dog tick (D. variabilis) were obtained from seven and two sites, respectively. Abundance of I. pacificus adults varied in clinical-case areas as well as in the comparison areas, and was not correlated significantly with spirochetal infection rates in this tick. Overall, spirochetes were detected in 1.4 and 1.0% of the adult I. pacificus collected from clinical case (n = 857) and comparison sites (n = 383), respectively, and in 0.8% of adult D. occidentalis (n = 253) from one comparison area. An additional 244 D. occidentalis adults from all other sites were tested with negative results. Five spirochetal isolates recovered from I. pacificus adults were identified as Borrelia burgdorferi with specific monoclonal antibodies. Seven of 10 patients interviewed reportedly contracted their infections in summer, and six presented with a history of tick bite. Nine patients owned or occasionally harbored one or more dogs, and at least one of the dogs contracted Lyme borreliosis at the same site as its owner. Clinical manifestations of the disease in human patients included erythema migrans (100%), arthritis/arthralgia (60%), neurologic abnormalities (60%), and cardiac involvement (20%). Four of nine patients whose sera were assayed by indirect immunofluorescence contained significant antibody titers to B. burgdorferi.