The lignan enterolactone, a phytoestrogen, may protect against hormone-dependent cancers and cardiovascular diseases. It is produced by the intestinal microflora from dietary precursors. Because of the pronounced impact of antimicrobials on the intestinal microflora, the authors examined whether serum enterolactone concentration is affected by previous use of oral antimicrobials. Enterolactone was measured by time-resolved fluoroimmunoassay in 2,753 Finnish men and women aged 25--64 years who participated in a cross-sectional national survey in 1997. Background information was collected by self-administered questionnaire, and data on antimicrobial treatment were gathered from the nationwide prescription database of the Social Insurance Institution. Serum enterolactone concentration was significantly lower in those who had used oral antimicrobials up to 12--16 months before serum sampling than in nonusers (16.4 vs. 19.3 nmol/liter). The concentration was associated with the number of treatments and the time from the last treatment. Modest differences were present between various antimicrobials. The authors' findings support the crucial role of gut microflora in the metabolism of lignans. Furthermore, recent use of antimicrobials should be considered when the association between serum enterolactone concentration and risk of chronic diseases is studied.