This study examined the test-retest measurement invariance of ratings of the ADHD symptoms. Mothers of a group of 217 clinic-referred children (boys = 169, girls = 49; age range = 7 to 17 years) completed ratings of the ADHD symptoms presented in a modified version of the Strengths and Weaknesses of ADHD-Symptoms and Normal Behavior Scale (SWAN-M) twice, 12 months apart. Confirmatory factor analysis (CFA) of their ratings at both time points provided more support for the bifactor model of ADHD [orthogonal general and specific factors for inattention (IA) and hyperactivity/impulsivity (HI) symptoms] over the 2-factor oblique model (factors for inattention IA and HI symptoms). For the bifactor model, measurement invariance testing using multiple-group CFA indicated support for configural and full scalar test-retest invariance when the chi-square difference test was applied. For metric invariance, all but one symptom showed invariance. When the differences in approximate fit indices were applied, there was support for full measurement invariance (configural, metric and scalar). The findings indicate that observed scores (based on maternal ratings of the SWAM-M), twelve months apart, are comparable as they have the same measurement properties. Several practical, theoretical, research and clinical implications of the findings are discussed.