In a prospective study we examined the efficacy of a standardized disinfection method in preventing the transmission of hepatitis B virus by the gastrointestinal fibrescope. Four HBSAg- and Dane particle-positive patients who have been endoscoped served as possible sources of hepatitis B virus infection. We cleaned the instrument with a 10% aldehyde liquid and used it immediately thereafter in 10 HBSAg-negative patients. Eight of them were followed-up for 7 months after the endoscopic procedure. None of them showed serological evidence of an endoscopically transmitted hepatitis B virus infection. It is concluded that disinfection with an aldehyde liquid is effective in preventing the transmission of hepatitis B virus by the fibrescope.