Objectives: The intracellular (i.c.) content of dopamine and its metabolites was measured in the peripheral blood lymphocytes (PBLs) of Parkinson’s disease (PD) patients and healthy controls. Methods: Catecholamine levels of PBLs were measured using capillary electrophoresis in healthy controls and PD patients receiving different doses of L-dihydroxyphenylalanine ( L-Dopa). Results: Higher i.c. dopamine content was found in lymphocytes from PD patients receiving a high dose of L-Dopa (700 ± 30 mg/day) as compared to lymphocytes from the healthy controls (p = 0.002) and from PD patients treated with a low dose of L-Dopa (400 ± 30 mg/day) (p = 0.022). The dihydroxyphenylacetic acid to dopamine ratio was significantly lower in the high-dose L-Dopa-treated PD patients than in the controls (p = 0.013). Conclusions: These findings suggest that the dopamine content and metabolism in the peripheral lymphocytes of PD patients are influenced by L-Dopa administration. This is the first study in which a dose-related effect of L-Dopa treatment was found in lymphocytes from PD patients.