This study was designed to investigate serum soluble interleukin-2 receptor (S-IL-2R), interleukin-2 (IL-2) and interleukin-6 levels (IL-6) in patients with either a positive or negative Borrelia burgdorferi serology. Serum samples from 101 individuals, divided in to five groups according to clinical symptoms and outcome of serology were analysed. Samples of cerebrospinal fluid (CSF) from nine of the individuals were also studied. The highest average serum S-IL-2R levels (1,180 +/- 1,140 U/ml) were found in patients with erythema migrans, the hallmark of Lyme borreliosis, followed by patients with symptoms closely related to Borrelia infection (900 +/- 1,200 U/ml) and with a strong positive serology. In two patients with central nervous system (CNS) involvement, increased levels of S-IL-2R of 920 and 620 U/ml respectively (normal value < 50 U/ml) were detected in the CSF. No statistically significant relationship between IgG or IgM antibody activity and serum S-IL-2R levels was found. Detectable levels of IL-2 were only found in three patients. Increased levels of IL-6 were found in sera from 14 patients. The highest concentration, 90 pg/ml (normal value < 10 pg/ml), was measured in a patient presenting with vasculitis. In conclusion, B. burgdorferi infection causes a moderate increase of serum S-IL-2R levels, although there is no relationship between the severity of the infection, as estimated by the antibody concentration or to serum IL-2 or IL-6 levels. Secondary complications of the infection, such as vasculitis, may cause an increased level of serum IL-6.(ABSTRACT TRUNCATED AT 250 WORDS)