The aims of the study were as follows: (1) to examine the width of the dental arches of patients with maxillary midline diastema and compare it with control group; (2) to investigate the impact of the width of upper dental arch on the width of diastema.Diagnostic orthodontic plaster models of 102 patients with permanent dentition were studied. Patients were divided into two groups: study group with diastema and control group without diastema. Patients with severe malocclusion, craniofacial diseases, hypodontia and microdontia and pa-tients with periodontal disease were excluded. The transpalatal width of palate, premolar and molar arch widths in Pont's points of upper and lower jaw were measured using digital calliper. The results were statistically analysed.Analysis showed a significant correlation between presence of diastema and premolar and molar width of the dental arches for both upper and lower jaw. Studied widths were larger in patients with diastema compared to the group without diastema. Analysis of the transpalatal width showed statistically significant differences between the study group and the control group. Analysis of widths of diastema and transpalatal widths showed that there was not statistically sig-nificant correlation.Patients with diastema had increased in size in both the premolar and molar width of the dental arches. Increase the width affect to both upper and lower dental arch. Patients with diastema also were characterised by often occurrence of normal or increased of the transpalatal width but the width of the diastema did not correlate with the width of the palate. (Folia Morphol 2018; 77, 2: 340-344).