Background: The Copenhagen Child Cohort, CCC 2000, was established to investigate developmental psychopathology prospectively from birth in a general population. Methods: A random sample of 211 children from the CCC 2000 was investigated when the children were 1 1/2 years of age. The prevalence and associates of mental health problems and psychopathology were studied by clinical and standardised strategies, including videotape recordings, parent interviews and the following instruments: The Child Behavior Check List 1 1/2-5 (CBCL 1 1/2-5), The Infant Toddler Symptom Check List (ITSCL), Checklist for Autism in Toddlers (CHAT), Bayley Scales of Infant Development II (BSID II), The Parent Child Early Relationship Assessment (PC ERA) and Parent Infant Relationship Global Assessment Scale (PIR-GAS). Results: Mental health problems according to International Classification of Diseases (ICD-10) and Diagnostic Classification Zero to Three (DC 0-3) diagnoses were found in 16-18% of 1 1/2-year-old children. Most common were disturbances of emotion, behaviour and eating and the DC 0-3 diagnosis of regulatory disorder. Parent-child relationship disturbances were found in 8%. High psychosocial risk was significantly associated with emotional and behavioural disorders (OR 3.1 95% (1.2-8.1)) and disturbed parent-child relationship (OR 5.0 95% (1.6-16.0)). The strongest association of risk was found between relationship disorders and emotional and behavioural disorders (OR 11.6 95% (3.8-37.5)). Conclusions: The prevalence and distribution of psychopathology in 1 1/2-year-old children seem to correspond to the distributions among older children. Disturbances in parent-child relationship have a key position in the risk mechanisms in early child psychopathology.