The work described aimed to evaluate the incidence of streptococcal bacteraemia in children undergoing bone marrow transplantation and receiving prophylaxis with penicillin V. From January 1991 to December 1993 oral penicillin V was administered as prophylaxis for streptococcal bacteraemia to patients undergoing bone marrow transplantation at G. Gaslini Children's Hospital, Genoa, Italy. The data were compared with those from a similar population receiving bone marrow transplantation from September 1984 to July 1990 and not receiving this kind of prophylaxis. Streptococcal bacteraemia was diagnosed in 7/17 (41%) episodes of bacteraemia observed in the period January 1991 to December 1993, while it accounted for 71% of all bacteraemias in the period from September 1984 to July 1990 and was especially frequent from January 1988 to July 1990, comprising 13/15 (87%) of observed bacteremias. The decrease of this complication observed after the beginning of the prophylaxis programme was statistically significant. Oral penicillin V is effective as prophylaxis of streptococcal bacteraemias in children receiving bone marrow transplantation in a centre with a high incidence of this complication.