Psoriasis is a chronic inflammatory disorder mediated by the immune system which is characterized by skin inflammation, epidermal hyperplasia and other co-morbidities. The measurement tool most often used to assess the severity and extension of psoriasis is Psoriasis Area and Severity Index (PASI) score. Malondialdehyde (MDA) is a biomarker in oxidative stress which functions as a mutagenic marker of lipid peroxidation, can damage DNA and cause cell death. Measurement of serum MDA levels is useful for predicting the level of oxidative stress in various chronic diseases, including psoriasis vulgaris. Purpose: To determine the correlation between serum malondialdehyde levels and Psoriasis Area and Severity Index (PASI) score in psoriasis vulgaris. Methods: Observational analytic study with a cross-sectional approach (cross-sectional) on 37 psoriasis vulgaris subjects and 37 healthy control subjects at Haji Adam Malik General Hospital Medan. Results: The majority of psoriasis vulgaris occurred in men (54.1%) with the most age group being 30-39 years. The mean of PASI score in the psoriasis vulgaris group was 7.37 ± 4.13. Serum malondialdehyde levels in the psoriasis vulgaris group were higher than in the control group (1.84 ± 0.38 nmol/ml vs 1.33 ± 0.19 nmol/ml). Spearman's correlation test showed a correlation coefficient (r) of 0.967 with p <0.001. Conclusion: There is a significant positive correlation between serum malondialdehyde levels and Psoriasis Area and Severity Index (PASI) score in psoriasis vulgaris.