Background: Platelet activation contributes to cardiovascular disease (CVD), the main complication of type 2 diabetes mellitus (T2DM) and pre-diabetic conditions. Mean platelet volume (MPV) is an easy-to-measure platelet parameter which has been associated with CVD. Objective: We sought to assess MPV, platelet distribution width (PDW), and platelet count in T2DM, impaired fasting glucose (IFG), impaired glucose tolerance (IGT), and metabolic syndrome (MS). Patients/Methods: Web-based literature search (PubMed, EMBASE and Web of Science) of studies published in English through June 2014 was performed to select case-control and cross-sectional studies that reported data on MPV, PDW, or platelet count in cases (subjects with T2DM, IFG, IGT, or MS) and noncases. Descriptive and quantitative information was extracted and within-study standardized mean difference (SMD) was estimated from means and standard deviations. SMDs across studies were synthesized using a random random-effects model and subgroup analyses were performed on pre-specified study-level characteristics. Results: Thirty-nine studies were included. Compared to controls, MPV was significantly higher in T2DM (SMD, 95% CI: 0.70, 0.50-0.91; N=24,245), IFG (0.14, 0.02-0.26; N=17,389) but not in MS (0.15, -0.24-0.55; N=14,990). PDW was wider in T2DM (0.93, 0.09-1.76; N=471). Platelet count resulted higher in IFG (0.18, 0.12-0.24; N=3,960) and MS (0.39, 0.01-0.78; N=4,070). Only two studies included IGT. Conclusions: Available data suggest that T2DM subjects tend to have higher MPV and PDW values, but nondifferent platelet count as compared to subject without T2DM. Whether and how these morphometric changes contribute to CVD of T2DM or can be used as CVD biomarker awaits further investigation.