The aim of this study was to examine the contribution of environmental factors to the pathogenesis of Alzheimer's disease by comparing environmental differences in twin pairs discordant for Alzheimer's disease. Seventy four twin pairs discordant for Alzheimer's disease were found by linking the Finnish twin cohort and the Hospital Discharge Register from years 1972-91. In 50pairs (25 monozygotic and 25 dizygotic pairs), both co-twins had responded to a questionnaire survey in 1975. Exposure differences were compared between these pairs. A reduced risk of Alzheimer's disease was significantly associated with a higher level of schooling (relative risk 0.3; 95% confidence interval 0.1-0.9, p=0.029). In addition, a reduced risk was suggestively associated with ambidextrousness or left handedness (p=0.083) and an increased risk with marriage (p=0.052), widowhood (p=0.074), and a history of cholelithiasis (p=0.071). In conclusion, a reduced risk of Alzheimer's disease was associated with a higher level of schooling.