In recent years, Streptococcus pneumoniae and Haemophilus influenzae have acquired resistance to some drugs; as such, a therapeutic strategy in which penicillins (PCs) are primarily used is currently recommended for the treatment of pediatric upper respiratory tract infections. In 2000, we changed the primary drug used for such infections in patients visiting our hospital from cephems (CEPs) to PCs. We examined the effect of this change on the detection rates of drug-resistant S. pneumoniae (DRSP) and ampicillin (ABPC)-resistant H. influenzae. 1. The subjects consisted of patients who had visited our hospital and received bacteriological examinations during 1999, 2001 or 2003. At the time of the examination, we questioned each patient regarding the medical institutions where they had received oral antibiotic therapy during the past year, how many antibiotic therapies they had received, and which antibiotics they had received. The number of patients who had only received antibiotics from our hospital was 37 out of 52 (71%) patients in 1999, 80 out of 110 (73%) patients in 2001, and 50 out of 71 (70%) patients in 2003. The number of therapies in which only PCs were used was significantly higher in 2001 and 2003 than in 1999. The number of therapies in which only CEPs were used was lower in 2001 and 2003 than in 1999. We believe that this trend reflects our therapeutic strategy. Furthermore, the antibiotic therapies that patients received at other hospitals were thought to have only a small influence on the detection rates of DRSP and ABPC-resistant H. influenzae. 2. The detection rate for DRSP was 30% in 1995 and 60% in 1999. However, the detection rate for DRSP was 37% in 2001 and 39% in 2003, representing a significant decrease from the detection rate in 1999. The detection rate for ABPC-resistant H. influenzae was 26% in 1995 and increased slightly to 36% in 1999. However, the detection rate for ABPC-resistant H. influenzae increased significantly from 36% in 1999 to 59% in 2001 and to 71% in 2003. The reason for this trend is that PCs are considered to be less effective than CEPs against H. influenzae, even though they are more effective than CEPs against S. pneumoniae. 3. We received the patients at our hospital who required hospitalization for the treatment of bacterial infections. In 1999, DRSP was detected in 2 of the 11 (18%) patients who required hospitalization, but no ABPC-resistant H. influenzae strains were detected. ABPC-resistant H. influenzae was detected in 3 of the 12 (25%) patients hospitalized in 2001 and in 4 of the 10 (40%) patients hospitalized in 2003. Thus, the detection rate for ABPC-resistant H. influenzae appears to be increasing. 4. Our findings suggest that the number of DRSP isolates may increase when treatment policies focused on CEPs are employed, while the number of ABPC-resistant H. influenzae isolates may increase when treatment policies are focused on PCs. 5. At present, guidelines focusing on the use of PCs for the treatment of pediatric upper respiratory tract infection are recommended, but standardizing the choice of antibiotics may cause the number of resistant bacteria to increase. When such guidelines are employed at medical institutions, attention should be constantly paid to trends in drug susceptibility. If an increase in one type of resistant bacteria is observed, flexible measures should be taken, without strict adherence to the guideline.