Common single nucleotide polymorphisms (SNPs) in the ACDC adiponectin encoding gene have been associated with insulin resistance and type 2 diabetes in several populations. Here, we investigate the role of SNPs −11,377C>G, −11,391G>A, +45T>G, and +276G>T in 2,579 French Caucasians (1,229 morbidly obese and 1,350 control subjects). We found an association between severe forms of obesity and −11,377C (odds ratio 1.23, P = 0.001) and +276T (1.19, P = 0.006). Surprisingly, alternative alleles −11,377G and +276G have been previously reported as risk factors for type 2 diabetes. Transmission disequilibrium tests showed a trend in overtransmission (56.7%) of a risk haplotype 1_(C)-1_(G)-1_(T)-2_(T) including −11,377C and +276T in 634 obesity trios (P = 0.097). Family-based analysis in 400 trios from the general population indicated association between obesity haplotype and higher adiponectin levels, suggesting a role of hyperadiponectinemia in weight gain. However, experiments studying the putative roles of SNPs −11,377C>G and +276G>T on ACDC functionality were not conclusive. In contrast, promoter SNP −11,391G>A was associated with higher adiponectin levels in obese children (P = 0.005) and in children from the general population (0.00007). In vitro transcriptional assays showed that −11,391A may increase ACDC activity. In summary, our study suggests that variations at the ACDC/adiponectin gene are associated with risk of severe forms of obesity. However, the mechanisms underlying these possible associations are not fully understood.