Purpose: To determine whether inflammatory reactions are involved in the pathogenesis of diabetic macular edema, we examined the relationship between diabetic macular edema and the ratio of T helper 1 (Th1) to T helper 2 (Th2) cells. Methods: Thirty-nine diabetic patients with diabetic retinopathy were evaluated at our hospital between February 2004 and February 2005. Blood samples were collected from each patient, and the ratio of CD4+ Th1 to Th2 cells (Th1/Th2) was determined by flow cytometry after fluorescent antibody staining for intracellular cytokines. Logistic regression analysis was used to determine the association of macular edema with age, gender, HbA1c level, interval after retinal photocoagulation and Th1/Th2 ratios. Differences in these parameters were also compared between patients with and without macular edema. Results: Logistic regression analysis showed that the Th1/Th2 ratios were significantly associated with macular edema (odds ratio = 0.838; p = 0.02), while other variables were not. The Th1/Th2 ratio was significantly lower in the patients with diabetic macular edema than in those without (p = 0.02; t test). Higher Th1/Th2 ratios tended to be associated with better visual acuity. Conclusions: Shifts in the balance of Th1/Th2 towards a predominance of Th2 may represent an exacerbating factor for diabetic macular edema, although a causal relationship has still not been definitively determined.