We report 2 cases of household transmission of pneumococcal pneumonia in isolation, associated with pandemic influenza (H1N1) 2009. A Chinese-American family consisting of a 47-year-old woman and her 16- and 9-year-old sons came to Japan at the beginning of July 2009. The day after their arrival, the woman noticed a high fever in the eldest son. The following day, the other family members also developed high fevers. Real-time reverse transcription polymerase chain reaction (RRT-PCR) analysis confirmed pandemic influenza A (H1N1) 2009 infection in all family members. After diagnosis, all patients were given oseltamivir at another hospital. Subsequently, they were admitted to our hospital and placed in isolation in accordance with the Japanese Infectious Disease Law. At the time of admission, all family members were in a stable condition. However, on the day after admission, the mother complained of a productive cough. Upon further investigation, a CT scan showed a consolidation shadow in the right middle lung lobe. After isolating Streptococcus pneumoniae from her sputum culture, we diagnosed bacterial pneumonia due to S. pneumoniae. In addition, both of her sons also developed fever and cough, and sputum obtained from her elder son indicated S. pneumoniae infection. All 3 were treated with ampicillin and their symptoms improved over a period of a few days. Although the role of Streptococcus pneumoniae in cases of novel influenza A (H1N1) remains unclear, these bacteria may have been responsible for many complications during past pandemic and non-pandemic influenza cases. Use of the 23-valent pneumococcal polysaccharide vaccine should be considered as a potential way to prevent pneumococcal pneumonia during future influenza pandemics.