Achillea species are widely used in folk medicine as antispasmodic, choloretic, antiphlogistic, antifungal and antibacterial agents, but little is known about their effects on the immune system and blood lipid profile. In this study, effects of the Achillea wilhelmsii methanolic extracts (ME) upon the lipid, haematological parameters and immune system were examined. ME was given at dose 75 mg/kg/day for 30 days to Sprague-Dawley rats fed with high-fat diet. Haematology, flow cytometry, clinical, biochemical and histopathological examinations were performed in the sacrificed animals. Significant decreases in triglyceride and very low density lipoprotein cholesterol, and increases in CD4 expression in lymphocytes and monocytes were observed. Assessment of alanin aminotransferase, aspartat aminotransferase activities, histopathological examinations and proinflammatory cytokine levels (IL-1 beta, IL-6, IL-10, TNF-alpha) showed neither liver damage nor inflammation induction. Beside the improvements in lipid profile by ME treatment without any liver damage and inflammation, the increase of CD4 expression both on lymphocyte and monocyte populations need further investigation.