In trematodiases, shape and size of the fluke eggs shed with faeces are crucial diagnostic features because of their typically reduced intraspecific variability. In fascioliasis, the usual diagnosis during the biliary stage of infection is based on the classification of eggs found in stools, duodenal contents or bile. The aim of the present study is to validate the identification of Fasciola species based on the shape and size of eggs shed by humans, characterizing their morphometric traits using a computer image analysis system (CIAS). The influence of both the geographical location and of the host (human and livestock) has been analysed. Coprological studies were carried out in fascioliasis human endemic areas, where only F. hepatica is present (the northern Bolivian Altiplano and the Cajamarca valley in Peru), and where F. hepatica and F. gigantica coexist (the Kutaisi region of Georgia, the Nile Delta in Egypt, and the Quy Nhon province in Vietnam). Classically, it is considered that at the abopercular end of the shell of Fasciola eggs there is often a roughened or irregular area. Nevertheless, results show that the frequency of the presence of this feature in F. hepatica is population-dependent, and therefore is not a pathognomonic criterion in diagnosis. The study reveals that eggs shed by humans show morphological traits different from eggs shed by animals. In humans, F. hepatica eggs are bigger and F. gigantica eggs are smaller than reported to date from livestock, and their measurements overlap when compared. The material analysed in this study shows that the size of eggs shed by humans from Georgia and Egypt corresponds to the F. hepatica morph, while the size of eggs shed by humans from Vietnam corresponds to the F. gigantica morph. Measurements of F. hepatica and F. gigantica eggs originating from humans and animals from sympatric areas overlap, and, therefore, they do not allow differential diagnosis when within this overlapping range. In this sense, the new results should aid clinicians since the application of the classic egg size range in human samples may lead to erroneous conclusions. Fasciolid egg size in human stool samples ought to be corrected in books and monographs related to medical parasitology and/or tropical medicine as well as in guides for clinicians and parasitic disease diagnosis analysts.