The following article shows that aspartame causes:
a) an increased incidence of malignant-tumor-bearing animals with a positive significant trend
b) an increase in lymphomas and leukemias with a positive significant trend
c) a statistically significant increased incidence, with a positive significant trend
d) an increased incidence of malignant schwannomas of peripheral nerves with a positive trend

For more information see:
First experimental demonstration of the multipotential carcinogenic effects of aspartame administered in the feed to Sprague-Dawley rats.