Since immunosuppressed patients are at higher risk of serious influenza virus infection than healthy subjects, we decided to study the serological effectiveness of influenza vaccination on renal transplant patients, despite the theoretical aspect that such treatment could induce glomerular lesions through an immunological process. Forty transplant patients aged from 20 to 50 years with well functioning renal graft and no febrile episode were studied. Blood samples were collected before the intramuscular injection of 0.5 ml of multivalent influenza vaccine (PASTEUR MERIEUX SERUM VACCINS), at one and at two months after the vaccination. Before vaccination, the antibody titers to influenza virus ranged from 0 to 1/20 and after vaccination from 1/20 to 1/320. One month after vaccination 17/40 (42.5%), 18/31 (58%) and 16/33 (48%) patients showed a four-fold or greater increase of serum influenza antibody titers to antigens A/H3N2, A/H1N1 and B, respectively. A similar response at two months in relation to the first month response rate after vaccination was found in 15/17 (88%), 18/18 (100%), and 15/16 (93%) of transplant patients for the above mentioned three antigens. Side-effects were observed in two of the studied patients. Serum creatinine and urine protein were not changed. Also acute graft rejection episodes were not observed. It is suggested that influenza vaccination is safe and serologically effective on renal transplant patients.