Background: Substance P (SP) is a nociceptive tachykinin which regulates the immune inflammatory reactions including monocyte chemoattractant protein 1 (MCP-1) production. Sequential second-eye cataract surgery patients often suffer more pain than the first one partly because of the MCP-1 increase in aqueous humor (AH). This study aims to illustrate whether SP is involved in sympathetic inflammatory responses in the contralateral eye in patients with or without type 2 diabetes. Methods: This prospective randomized clinical study included 51 cataract patients (22 with diabetes and 29 without). Bilateral sequential cataract surgeries were conducted with 1-day or 1-week interval randomly. More than 100 µl of AH were obtained before surgery and stored for later analysis using magnetic Luminex assays to detect interleukin (IL)-1β, IL-1ra,IL-6, IL-8, IL-10, MCP-1, vascular endothelial growth factor, spinal macrophage inflammatory protein (MIP-1a), interferon-inducible protein 10 (IP-10), regulated on activation, normal T expressed and secreted (RANTES), as well as the enzyme-linked immunosorbent assay for SP. Results: Among the 4 groups with 1-day or 1-week intervals, and of the 51 cataract patients with or without diabetes, no significant differences were found in age, sex distribution, the R/L ration of the first surgery eye, or the lens nucleus hardness (P ≥ 0.802). Over 100 µl of AH samples were collected safely in all cases without intraoperative complications. SP and MCP-1 levels were both increased significantly in the second eye of the diabetic patients with 1-day and 1-week interval ( P ≤ 0.040). The SP increase in the second eye of the diabetic patients were significantly higher than that of the patients without diabetes ( P ≤ 0.030) both in the groups with 1-day and 1-week interval. Similarly, the MCP-1 increase was significantly higher in the diabetic patients in the group with 1-week interval (P = 0.042). Conclusions: Substance P and MCP-1 productions elevated in the AH of the contralateral eye after the first-eye cataract surgery in diabetic patients, indicating that SP and MCP-1 were involved in the sympathetic inflammatory responses. Diabetic patients are susceptible to noninfectious inflammation after cataract surgery, and perceive more pain in the second-eye phacoemulsification.