Background: Biologic therapies (BTs), etanercept, infliximab, adalimumab, and ustekinumab, are generally well-tolerated and safe agents in psoriasis management. Objectives: To determine the overall effect of BTs on peripheral blood eosinophil count (PBEc) and percentage (PBEp), peripheral blood basophil count (PBBc) and percentage (PBBp), white blood cell count (WBCc), erythrocyte sedimentation rate (ESR), and serum C-reactive protein (s-CRP) level during a 3-year follow-up in patients with psoriasis. Methods: This retrospective cohort study included 200 patients (116 men; 84 women) treated continuously with BTs for 3 years for plaque-type, pustular, or nail psoriasis. Patient data were reviewed from medical charts. During routine laboratory investigation at baseline and every 3 months thereafter up to 3 years, the PBEp, PBEc, PBBp, PBBc, WBCc, ESR, and s-CRP level were monitored. Generalized estimating equations were used to compare consecutive data. Results: Seventy patients received infliximab (35%); 34 (17%), etanercept; 44 (22%), adalimumab; and 52 (26%), ustekinumab. The mean PBEp and PBEc significantly increased starting from 3 months after BT (both p<.001). The mean PBEp and PBEc significantly increased during follow-up compared with the baseline values (PBEp (%): 1.49 +/- 0.1 (1st month) vs. 2.29 +/- 0.14 (3rd month), p<.001 and 1.49 +/- 0.1 (1st month) vs. 2.17 +/- 0.18 (36th month), p=.004; PBEc (x10(3)/mu L): 115.80 +/- 6.32 (1st month) vs. 174.9 +/- 10.08 (3rd month), p<.001 and 115.80 +/- 6.32 (1st month) vs. 162.9 +/- 12.86 (36th month), p<.001). However, the mean PBBp, PBBc, WBCc, ESR, and s-CRP level did not change significantly. Conclusions: The PBEc and PBEp increase with BTs up to 3 years in patients with psoriasis. This increase is observed at as early as 3 months of BT and maintained thereafter.