Background: Periodontal disease and diabetes mellitus are 2 most common encountered chronic diseases. The occurrence, appearance, and severity of the periodontium tissue conditions vary during adolescence. Also, there is controversy about the relationship between diabetes mellitus and increased periodontal disease breakdown during puberty. This study aimed to assess the periodontal parameters and oral hygiene in diabetic and nondiabetic adolescents in city of Zanjan. Methods: In this case and controls study, 140 diabetic (case) and 140 nondiabetic (control) adolescents were compared. The participants were matched by age, sex, and socioeconomic status using the group matching method. Clinical attachment loss, periodontal pocket depth, gingival index, periodontal index, and bleeding on probing were measured for the 6 Ramfjord index teeth. Data were analysed using descriptive statistics, chi-square test, t test, and one-way ANOVA using SPSS version 18. Results: The mean age of the diabetic and nondiabetic group was 16.7±1.6 and 17.1±0.30 years, respectively. Duration of diabetes was 2.15 ±0.86 years. Mean clinical attachment loss, periodontal pocket depth, gingival index, and bleeding on probing in the patient group were 7.4, 1.68, 12.9, and 97.7%, respectively. All these indices in diabetic adolescents were significantly greater than in nondiabetics. Only the PI was poor in both groups and no significant difference was noted in this respect between the 2 groups. Conclusion: Both diabetic and nondiabetic adolescents had equally poor oral hygiene. Gingivitis and periodontitis indices were significantly poorer in diabetic patients.