This study aims to assess the impact of a brief patient group education intervention in people with type 2 diabetes mellitus. The sample, 193 people with type 2 diabetes mellitus who were patients at the diabetic clinic of a primary health care setting in Attica, was assigned to two groups, intervention (138 individuals) and control group (55 individuals). The intervention group, instead of receiving standard care, attended a structured group educational programme using Conversation Maps while the control group had standard care. The main outcome measures were HbA1c, body mass index (BMI), triglycerides, high density lipoprotein (HDL), low density lipoprotein (LDL), measured at baseline and 6 months after the intervention. After 6 months, significant differences in changes of the studied parameters were observed between the groups in HbA1c, 1.4 (95% CI: 1.1, 1.7), (P < 0.001) and HDL -4.4 (95% CI: -8.1, -0.8), (P < 0.001) in favour of group education. Within the groups, the intervention group presented a significant reduction in HbA1c, -0.6 (95% CI: -0.8, -0.3), (P < 0.001), in BMI, -0.7 (95% CI: -0.9, -0.1), (P = 0.007), in triglycerides, -21.1 (95% CI: -47.1, -9.9), (P = 0.003) and LDL, -10.0 (95% CI: -22.5, -5.7), (P < 0.001), while in the control group significant reductions were observed in HbA1c, -0.5 (95% CI: -0.5, -0.3), (P = 0.003), in HLD, -4.1 (95% CI: -7.7, -2.3), (P = 0.001) and in LDL, -9.4 (95% CI: -19.9, -3.9), (P = 0.018). Group-based patient education using Conversation Maps for people with type 2 diabetes is more effective, compared with individual education, in diabetes self-management.