Hepatitis B virus (HBV) markers are found with high frequency in immunocompromised individuals. In order to find out if this is also true for the hepatitis C virus (HCV), we have analyzed a group (G.1) of 46 patients (pts.) with Down syndrome, situation known to be associated with immunodepression G. 1. We compared them with a G. of 310 mentally retarded pts. without Down syndrome G. 2 and without evidence of immunological disfunction. All of them were studied for infection with HBV. All pts. in G. 1 and G. 2 were also tested for HCV. The pts. have been hospitalized in a specialized medical institution for mentally retarded on a long term basis and were followed during 1 year. Finally G 3 was composed of 5454 voluntary blood donors. METHODS In all pts. search for HBV infection markers (anti-HBc, HBsAg, HBeAg by EIA test and HBV-DNA by nucleic acids hybridization) were performed. Search for HCV markers was done by a second generation EIA kit (Abbott Hepatitis C (rDNA) (Antigen). RESULTS HBsAg was found to be positive in 12/46 (26%) of G. I and 25/310 (8%) of G. II (p < 0.001). HBeAg was positive in 8/12 (67%) of G. I and in 2/25 (8%) of G. II (p < 0.001). All HBeAg positive pts. had elevated values of DNA-HBV. In G. I, 4/12 (33%) pts. lost HBeAg during the observation period, one of them remained HBV-DNA positive and none become HBsAg negative.(ABSTRACT TRUNCATED AT 250 WORDS)