Our aim was to study the protein expression profiles of placenta obtained from lean and obese pregnant women with normal glucose tolerance at the time of term Caesarean section. We used two-dimensional difference gel electrophoresis (2D-DIGE), utilising narrow-range immobilised pH gradient strips that encompassed the broad pH range of 4-5 and 5-6, followed by MALDI-TOF mass spectrometry of selected protein spots. Western blot and quantitative RT-PCR (qRT-PCR) analyses were performed to validate representative findings from the 2D-DIGE analysis. Eight proteins were altered (six down-regulated and two up-regulated on obese placentas). Annexin A5 (ANXA5), ATP synthase subunit beta, mitochondria (ATPB), brain acid soluble protein 1 (BASP1), ferritin light chain (FTL), heterogeneous nuclear ribonucleoprotein C (HNRPC) and vimentin (VIME) were all lower in obese patients. Alpha-1-antitrypsin (A1AT) and stress-70 protein, mitochondrial (GRP75) were higher in obese patients. Western blot analysis of ANXA5, ATPB, FTL, VIME, A1AT and GRP75 confirmed the findings from the 2D-DIGE analysis. For brain acid soluble protein 1 and HNRPC, qRT-PCR analysis also confirmed the findings from the 2D-DIGE analysis. Immunohistochemical analysis was also used to determine the localisation of the proteins in human placenta. In conclusion, proteomic analysis of placenta reveals differential expression of several proteins in patients with pre-existing obesity. These proteins are implicated in a variety of cellular functions such as regulation of growth, cytoskeletal structure, oxidative stress, inflammation, coagulation and apoptosis. These disturbances may have significant implications for fetal growth and development.