The handedness of 942 subjects (305 tertiary students, 591 of their siblings and 46 of their children) was ascertained by a 14-item questionnaire. The mothers of the subjects supplied information about maternal age at birth, birth weight and the presence or absence of twelve conditions likely to be associated with birth stress for each subject. No increase in left handedness was found among fourth or later born children. A significant decrease occurred in first-borns of both sexes, although these had more stressful births than the other subjects. No relationship between maternal age, birth weight or reported birth stress and left handedness was found. Thus the hypothesis that birth stress is a major cause of left handedness in normal subjects was not supported. Acknowledgements. We are particularly grateful to the students from the Institute for Early Childhood Development and their families, who provided the data for this study. We also wish to thank Dr John L. Bradshaw for his very constructive comments during the preparation of this paper.