Objectives An observational study to evaluate the relationship between serum concentrations of adalimumab and disease activity in patients receiving long-term adalimumab treatment for psoriatic arthritis. Methods Serum adalimumab and adalimumab antidrug antibodies were quantified by enzyme linked immunosorbent assay. Disease activity was assessed using Disease Activity Score (44 joint measures). Serum C-reactive protein was quantified using standard methods. Results A total of 30 patients were recruited. There were significant inverse correlations between serum adalimumab concentration and serum C-reactive protein (CRP) concentration [ r = −0.43], the number of tender joints ( r = −0.4), and Disease Activity Score (DAS44)-CRP ( r = −0.36). Mean serum adalimumab levels were significantly higher in patients with DAS44-CRP <1.6 than in patients with DAS44-CRP ≥1.6. Conclusions Serum adalimumab could be an important tool that may improve the management of psoriatic arthritis in patients responding to long-term treatment.