Iron is a critical co-factor for several enzymes and is known to regulate gene expression in many pathogens. Streptococcus pneumoniae (pneumococcus) normally colonizes the upper respiratory mucosa, which is an iron-restricted environment. In contrast, during bacteremia available iron from heme and non-heme proteins potentially increases. In iron-depleted medium pneumococcal strain TIGR4 showed reduced growth, however, addition of several physiological iron sources restored growth. Gene expression of selected known and putative pneumococcal virulence factors was analyzed by quantitative RT-PCR in response to iron sources in vitro and during colonization, pneumonia, and bacteremia in a mouse model. Change in mRNA levels relative to transcription in iron-depleted medium was reported. In presence of iron sources, transcription of cps4A, zmpA, pavA, hemolysin and a putative exfoliative toxin was significantly increased, but nanB was suppressed. Hemoglobin at physiological concentration repressed ply and pspA expression. Ferritin, an acute phase protein, increased expression of an iron ABC transporter and repressed expression of a bacterial non-heme iron-containing ferritin. Transcription of cps4A, nanB, hemolysin, and a putative exfoliative toxin were significantly up-regulated during pneumonia and bacteremia, while mRNA of pavA and non-heme ferritin were expressed at higher levels during pneumonia and carriage. An iron ABC transporter was most up-regulated during bacteremia, while pspA and ply were expressed only in pneumonia. Transcription of zmpA was elevated during both pneumonia and bacteremia. These findings suggest that a subset of virulence genes in pneumococci is differentially regulated in response to the quantity and form of iron sources available in a host.