The current study compared the prevalence of cognitive and psychopathological impairments among 24 preschool children with Williams syndrome (WS) (aged 2.20 to 5.97years) and 53 controls without WS and screened for developmental or psychological diagnoses (aged 2.21 to 5.89years) matched on chronological age and sex distribution. Associations between sex, chronological age, early development and psychopathology were also investigated. The Child Behavior Checklist-Preschool Version (CBCL) and the Mullen Scales of Early Learning were administered. Higher reported rates of Attention Problems, Emotional Reactivity, Somatic Complaints, Withdrawal, Affective Problems and Total Problems were found in preschool children with WS. There were no significant group differences in prevalence rates of all other CBCL domains. Attention Problems were the most prevalent psychopathology in preschool children with WS (33% falling in the clinically significant range), followed by Affective Problems (29% in clinically significant range), then Anxiety Problems (17%) or Attention Deficit/Hyperactivity Problems (17%). Among children without WS, the highest prevalence rates of psychopathology were for Attention Problems (4% falling in the clinically significant range), Aggressive Behaviour (4%), Sleep Problems (4%) and Oppositional Defiant Problems (4%). There were no significant associations between sex or chronological age and CBCL-reported psychopathology for either group. In addition, there were no significant relationships between CBCL ratings and verbal ability, nonverbal ability or overall developmental level in either group. Findings highlight variations in the pattern of psychopathology among preschool children with WS compared to those without WS, which needs to be considered in clinical management and future research.