Recently, we developed a computer-adaptive test (CAT) for assessing health-related quality of life (HRQoL) in children and adolescents: the Kids-CAT. It measures five generic HRQoL dimensions. The aims of this article were (1) to present the study design and (2) to investigate its psychometric properties in a clinical setting. The Kids-CAT study is a longitudinal prospective study with eight measurements over one year at two University Medical Centers in Germany. For validating the Kids-CAT, 270 consecutive 7- to 17-year-old patients with asthma (n = 52), diabetes (n = 182) or juvenile arthritis (n = 36) answered well-established HRQoL instruments (Pediatric Quality of Life Inventory (TM) (PedsQL), KIDSCREEN-27) and scales measuring related constructs (e.g., social support, self-efficacy). Measurement precision, test-retest reliability, convergent and discriminant validity were investigated. The mean standard error of measurement ranged between .38 and .49 for the five dimensions, which equals a reliability between .86 and .76, respectively. The Kids-CAT measured most reliably in the lower HRQoL range. Convergent validity was supported by moderate to high correlations of the Kids-CAT dimensions with corresponding PedsQL dimensions ranging between .52 and .72. A lower correlation was found between the social dimensions of both instruments. Discriminant validity was confirmed by lower correlations with non-corresponding subscales of the PedsQL. The Kids-CAT measures pediatric HRQoL reliably, particularly in lower areas of HRQoL. Its test-retest reliability should be re-investigated in future studies. The validity of the instrument was demonstrated. Overall, results suggest that the Kids-CAT is a promising candidate for detecting psychosocial needs in chronically ill children.