Roe deer (Capreolus capreolus) were investigated for their value as sentinel animals for Lyme borreliosis in the Netherlands. Serum was obtained from 114 roe deer, and 513 Ixodes ricinus, predominantly females (72%), were obtained from 47 animals (41%). The polymerase chain reaction was used to detect DNA of Borrelia burgdorferi sensu lato in a total of 190 ticks, comprising 106 engorged ticks and 84 non-engorged ticks. Borrelia DNA was detected in 24 engorged ticks (23%) and 26 non-engorged ticks (31%). This difference was not significant (P = 0·25). Four species of B. burgdorferi sensu lato were identified in the ticks: B. burgdorferi sensu stricto, Borrelia garinii, Borrelia afzelii and group VS116. B. afzelii was most commonly found and present in 13 mixed infections, and in 28 single infections. Fifteen sera (13%) contained antibodies to Borrelia spp. Ticks are more appropriate sentinel animals for Lyme borreliosis than roe deer, an important host for I. ricinus. Although the viability of borrelia spirochaetes in engorged ticks collected from roe deer was not assessed, a bloodmeal taken from roe deer did not eliminate borrelia spirochaetes from the tick. The relevance of this finding for transovarial transmission of borrelia spirochaetes in ticks is discussed.