Twenty asthmatic children and 48 patients with severe psychomotor retardation were inoculated intranasally with trivalent cold-adapted recombinant (CR) influenza vaccine containing CR-125 (H1N1), CR-159 (H3N2) and CRB-117 (B). The vaccinees were mostly seropositive. Severe adverse reactions or asthmatic attacks were not observed, but 7 (15%) of 48 vaccinees with severe psychomotor retardation developed mild to moderate fever. Significant antibody responses in hemagglutination-inhibition tests were demonstrated in 33 (49%) vaccinees to CR-125, 20 (29%) to CR-159 and 8 (12%) to CRB-117. Two nosocomial outbreaks of influenza were observed in the subsequent winter. During an outbreak with H3N2 in one ward of severe psychomotor retardation patients, 2 (11%) of 18 vaccinees became infected compared with 10 (48%) of 21 placebo controls in the same ward (P < 0.05). In the other outbreak, with influenza B virus, 2 (14%) of 14 vaccinees and 13 (52%) of 25 controls in the ward for asthmatic children were infected (P < 0.05). The results indicate that trivalent CR vaccine is safe and effective against nosocomial outbreaks of influenza.