Between September 1991 and April 1993 the hepatitis B vaccination with recombinant hepatitis B vaccine was administered in 41 cancer patients following first diagnosis. All patients were under 16 year of age, with negative hepatitis B virus (HBV) serology and normal hepatic function. They received 40 micrograms of vaccine by injection into the deltoid muscle at 0, 1, and 2, months, with a fourth dose planned at the 4th month for nonresponders. At 1 year a booster dose was given. All the patients began vaccination within 1 month following diagnosis, and periodic serologic follow-up was performed immediately after each vaccination and also in the 6th, 9th, and 12th months after vaccination. Patients with production of anti-HBs at a titer equal to or greater than 10 mIU/L were considered seropositive. The seroconversion rates were 12.4%, 21.9%, 41.0%, and 48.7% after the first, second, third, and fourth monthly doses, respectively. Seroconversion rates were 56.0% at 6 months, 67.5% at 9 months, and 70.5% at 12 months. Geometric mean antibody titers were 212 and 373 mI U/L at 9 and 12 months, respectively. No serious side effects were observed. HBV vaccination is recommended for pediatric cancer patients.