The aim of this prospective study was to assess the frequency of thrombocytopenia and abnormal liver function tests in the initial phase of tick-borne encephalitis. In 1994, 248 adult patients presented at the Department of Infectious Diseases, University Medical Centre Ljubljana, with acute lymphocytic meningitis or meningoencephalitis and serologically confirmed tick-borne encephalitis virus infection. In 180/248 (72.6%) patients, typical biphasic course of the illness was found and 28/180 (15.6%) patients with biphasic course were examined in both phases of the illness. In 20 out of these 28 (71.4%) patients, thrombocytopenia and leukopenia were found initially, in one (3.6%) patient only leukopenia was recorded and three (10.7%) patients had thrombocytopenia without leukopenia. In four (14.3%) patients leukocyte and thrombocyte values were within the normal range. The lowest leukocyte number was 1.4 x 10(9)/l and the lowest recorded thrombocyte number was 60 x 10(9)/l. Abnormal liver function tests were discovered in four out of 18 patients tested (22.2%). In conclusion, in the initial phase of tick-borne encephalitis thrombocytopenia and abnormal liver function tests may be found. Thrombocytopenia is a common finding with a frequency similar to that of well-known leukopenia, while abnormal liver function tests are relatively rare.