Many patients with psoriasis fail to respond to biologic drugs either initially or lose response over time, the latter having predominantly been linked to low circulating drug levels. We examined how serum drug levels varied over three treatment cycles of stable maintenance therapy with either adalimumab or infliximab among a total of 28 patients with psoriasis (22 men, mean age 48.6 years, mean treatment time 6.2 years) and whether there was an association with various patient-specific factors. The range for all concentrations was 1.1 to 24.3 μg/mL for adalimumab and 0.0 to 180.6 μg/mL for infliximab. There was a consistent inverse association between body mass index (BMI) and trough and maximum serum concentrations of adalimumab (P < .05 for all comparisons) and a positive, less consistent, association between age and maximum serum concentration of infliximab (P < .05 for both comparisons). Patient-specific factors, such as BMI and age, can help predict fluctuations in serum concentrations of biologics used for psoriasis.