The main goal of this study is to contribute to the understanding of the trajectory of comorbid disorders associated with obsessive-compulsive disorder (OCD) according to the first manifested psychiatric disorder and its impact in the clinical course of OCD and subsequent psychiatric comorbidities. One thousand and one OCD patients were evaluated at a single time point. Standardized instruments were used to determine the current and lifetime psychiatric diagnoses (Structured Clinical Interview for DSM-IV Axis I and for impulse-control disorders) as well as to establish current obsessive-compulsive, depressive and anxiety symptom severity (Yale-Brown Obsessive-Compulsive Scale; Dimensional Yale-Brown Obsessive-Compulsive Scale, Beck Depression and Anxiety Inventories and the OCD Natural History Questionnaire). To analyze the distribution of comorbidities according to age at onset Bayesian approach was used. Five hundred eight patients had the first OC symptom onset till the age of 10 years old. The first comorbidity to appear in the majority of the sample was separation anxiety disorder (17.5%, n=175), followed by ADHD (5.0%, n=50) and tic disorders (4.4%, n=44). OCD patients that presented with separation anxiety disorder as first diagnosis had higher lifetime frequency of post-traumatic stress disorder (p=0.003), higher scores in the Sexual/Religious dimension (p=0.04), Beck Anxiety (p<0.001) and Depression (p=0.005) Inventories. OCD patients that initially presented with ADHD had higher lifetime frequencies of substance abuse and dependence (p<0.001) and worsening OCD course (p=0.03). OCD patients that presented with tic disorders as first diagnosis had higher lifetime frequencies of OC spectrum disorders (p=0.03). OCD is a heterogeneous disorder and that the presence of specific comorbid diagnoses that predate the onset of OCD may influence its clinical presentation and course over the lifetime.