In scientific literature, several estrogenic compounds are reported to induce responses in vitro that are significantly higher than that of estradiol (E2). These supramaximal (SPMX) estrogenic effects do not occur consistently and seem to differ depending on the cellular models applied. This study analyzes the possible underlying causes, mechanisms, and drivers for SPMX estrogenic effects in in vitro functional assays reported in the peer-reviewed literature. For the 21 natural and industrial chemicals identified as SPMX inducers, the culture and exposure conditions varied greatly among and between the assays. Detailed information on assay characteristics, however, sometimes lacked. Diethylstilbestrol, genistein, and bisphenol A were selected to build a database. The meta-analysis revealed that the occurrence of SPMX effects could be related to a number of specific assay characteristics: (1) the type of serum used to supplement the exposure medium, (2) the end point used to quantify the estrogenic potency (endogenous or transfected), (3) the number of estrogen response elements, and (4) and the promoter's nature. An SPMX response was not reported for expression of endogenous genes, assays that used African green monkey kidney (COS-1) cell line or with chloramphenicol transferase as the reporter gene. There were no indications that solvent concentration in culture, exposure period, or cell model influenced the occurrence of an SPMX effect. It is important to understand the mechanism behind this phenomenon because in vitro assays for estrogenicity are used extensively to characterize and quantify the estrogenic potency of compounds, mixtures and environmental extracts.