We investigated the effects of Fascioloides magna infection on the serum biochemistry values of the naturally infected red deer population in eastern Croatia. The investigation was performed on 47 red deer with F. magna infection confirmed patho-anatomically in 27 animals (57.4%). Fibrous capsules and migratory lesions were found in 14 deer while only fibrous capsules without migratory lesions were found in 13 deer. In 13 deer both immature and mature flukes were found, in 5 deer only immature flukes were found and in 9 deer only mature flukes were found. Fascioloides magna infected deer with fibrous capsules and migratory lesions had significantly higher values for lactate dehydrogenase (LDH), glutamate dehydrogenase (GLDH) and globulin, and lower values for albumin/globulin ratio and glucose compared to uninfected deer. Fascioloides magna infected deer with fibrous capsules without the presence of migratory lesions had higher values for alanine aminotransferase (ALT) and globulin, and lower values for albumin/globulin ratio and glucose, than the uninfected deer. The number of immature flukes was positively correlated with values of aspartate aminotransferase (AST), γ-glutamyltransferase (GGT), LDH, GLDH, urea and triglycerides. The number of migratory lesions was positively correlated with GGT, GLDH, globulin and urea values. The creatinine value was positively correlated with the number of mature flukes. The trial showed that F. magna infection causes significant changes in serum biochemistry. Moreover, these changes do not completely resemble changes following F. hepatica infection. Further investigation of changes in liver enzymes and other serum metabolites in controlled, experimentally induced fascioloidosis in red deer is needed to better understand the pathogenesis of F. magna.