Between May and December 1998, tick-associated febrile illness was prospectively studied in Southeast Sweden in order to assess the occurrence of human granulocytic ehrlichiosis (HGE). Inclusion criteria were fever ( ≥ 38.0°C), with or without headache, myalgia or arthralgia in patients with an observed tick bite or tick exposure within 1 month prior to onset of symptoms. Patients with clinical signs of Lyme borreliosis were included. Of the 27 patients included, we identified 4 cases of HGE. Three of the patients had coinfection with Lyme borreliosis, which presented as erythema migrans. All 27 patients presented with a 2-5 d history of fever. None of the clinical signs or laboratory parameters monitored was helpful in predicting ehrlichiosis in this group with tick-associated fever conditions. Within the HGE-negative group (n = 23), 12 patients had clinical or laboratory signs of Lyme borreliosis. For 11 patients, the aetiology of the fever remained unclear. Our results suggest that HGE is common in tick-infested areas of Southeast Sweden, and may occur as a coinfection of Lyme borreliosis. Granulocytic ehrlichiosis should be suspected in patients who present with tick-associated fever, with or without erythema migrans. Ehrlichia serology and PCR should be employed to confirm the diagnosis.