A live-attenuated dengue 2 vaccine (strain 16681 PDK 53) developed at Mahidol University, Thailand was evaluated for safety and immunogenicity by administering 10(4) p.f.u. subcutaneously to ten flavivirus non-immune American volunteers. The vaccine was safe; there were no serious adverse reactions. Eight recipients experienced no or mild side effects. One recipient reported headaches on 7 separate days. One volunteer, who had a fracture of the humerus 1 day after vaccination requiring surgical repair, experienced generalized malaise with fever (maximum temperature = 38.9 degrees C), headache, eye pain and myalgia lasting less than 24 h. The vaccine was highly immunogenic; all recipients developed neutralizing antibody that persisted for two years.