Human granulocytic Ehrlichiosis (HGE) is a newly emerging acute febrile illness which is likely transmitted by ticks of the Ixodes ricinus/I. persulcatus complex. First seroepidemiological surveys on the prevalence of HGE antibodies, detection of DNA of granulocytotropic Ehrlichiae in I. ricinus and one case of HGE from Slovenia confirmed by serology and PCR (polymerase chain reaction) suggest that HGE might exist all over Europe. The purpose of the present study was a) to determine the prevalence of antibodies against the HGE agent in sera collected from persons at high risk for exposure to I. ricinus with that of a control population and b) to determine the prevalence of granulocytic Ehrlichiae in I. ricinus ticks from Southern Germany. We studied sera from 150 forestry workers and 105 patients with an established diagnosis of Lyme disease as tick-exposed populations. Sera from 103 healthy blood donors without a history of known tick bites served as controls. A significantly higher prevalence of HGE antibodies (P < or = 0.01) was present among patients with Lyme borreliosis (12 of 105 were positive; 11.4%) and forestry workers (21 of 150 were positive; 14%) compared to blood donors (2 of 103 were positive; 1.9%). Furthermore, 510 adult and nymphal I. ricinus were investigated by PCR for the presence of granulocytic Ehrlichiae with primers specific for the E. phagocytophila group. In eight (1.6%) of the investigated ticks the expected amplification product was detectable, indicating a low prevalence of infected ticks especially when compared with B. burgdorferi. The presented data strongly suggests that the HGE agent or a closely related organism exists in Southern Germany and therefore HGE should be considered in the differential diagnosis of febrile illnesses. However, final evidence can be provided only after isolation of the organism from patients.