Huntington's disease (HD) is caused by expansion of a glutamine repeat in huntingtin. Mutant huntingtin contains 36-55 repeats in adult HD patients and >60 repeats in juvenile HD patients. An N-terminal fragment of mutant huntingtin forms aggregates in neuronal nuclei in the brains of transgenic mice and HD patients. Aggregation of expanded polyglutamine is thought to be a common pathological mechanism in HD and other glutamine repeat diseases. It is not clear how the length of the repeats is correlated with formation of protein aggregates. By expressing a series of huntingtin constructs encoding various glutamine repeats (23-150 units) in cultured cells we observed N-terminal fragments of huntingtin (amino acids 1-67 and 1-212), but not full-length huntingtins, with glutamine repeats >/=66 units formed protein aggregates. Huntingtin aggregation was not induced when the repeat was </=49 units and was markedly promoted by very long repeats >/=120 units. This study suggests that various N-terminal fragments of mutant huntingtin can form aggregates and that aggregation is prompted by lengthening the glutamine repeat.