The relationship between nonverbal intelligence (spatial reasoning) and manual proficiency was studied in male and female left-handers. Manual proficiency was assessed by measuring the speed and accuracy in performing the dot-filling test. Nonverbal intelligence was assessed by Cattle's Culture Fair Intelligence Test. Females were found to be better than males in left-hand performance, and were more lateralized than males in manual proficiency. No significant sex difference could be established in right-hand performance. The right-hand skill was positively correlated to test intelligence (p less than 0.001). There was a negative correlation between the left- minus right-hand skill difference and the scores for nonverbal intelligence (p less than 0.001). The left-hand skill did not show any correlation with nonverbal IQ. It was concluded that the left-hemisphere is of utmost importance even for nonverbal intelligence; strong cerebral lateralization is disadvantageous for spatial reasoning.