Background: The differentiation hypothesis posits that the underlying liability distribution for psychopathology is of low dimensionality in young children, inflating diagnostic comorbidity rates, but increases in dimensionality with age as latent syndromes become less correlated. This hypothesis has not been adequately tested with longitudinal psychiatric symptom data. Methods: Confirmatory factor analyses of DSM-IV symptoms from seven common Axis I syndromes--major depression, generalized anxiety, separation anxiety, social anxiety, attention deficient hyperactivity, conduct, and oppositional defiant disorders--were conducted longitudinally, from ages 9 to 16, using the general-population Great Smoky Mountains Study sample. Results: An eight-syndrome model fit well at all ages, and in both genders. It included social anxiety, separation anxiety, oppositional defiant, and conduct syndromes, along with a multidimensional attention deficit-hyperactivity syndrome (i.e., inattention, hyperactivity, and impulsivity) and a unidimensional major depression/generalized anxiety syndrome. A high degree of measurement invariance across age was found for all syndromes, except for major depression/generalized anxiety. Major depression and generalized anxiety syndromes slightly diverged at age 14-16, when they also began to explain more symptom variance. Additionally, correlations between some emotional and disruptive syndromes showed slight differentiation. Conclusions: Marked developmental differentiation of psychopathology, as implied by the orthogenetic principle, is not a prominent cause of preadolescent and adolescent psychiatric comorbidity.