The aim of this pilot study was to assess a humoral response to influenza vaccine in 9 women with breast cancer. In the epidemic season 1998/1999 the patients received a single 0.5-ml dose of split influenza vaccine (Vaxigrip, Pasteur Merieux). Humoral response was measured by the hemagglutinin inhibition test in sera collected before vaccination and 1 month after vaccination. All results were compared with a control group of 19 healthy vaccinated women. The mean 'fold' increases ranged from 12.0 to 22.2 in patients with breast cancer and from 10.5 to 29.2 in healthy women. After vaccination, protection rates ranged from 44.4% to 88.9% and 63.2% to 94.7%, respectively. Response rates were between 44.4% and 88.9% in women with cancer and between 63.2% and 78.9% in the control group. After vaccination, antihaemagglutinin antibody titers were significantly higher than the prevaccination titers. During the whole study there were no statistically significant differences in humoral response between patients with breast cancer and healthy women. The results of the present study clearly show that women with breast cancer, including those undergoing chemotherapy, were able to develop a good serological response to influenza vaccine.