Background In patients with low-grade glioma (LGG), the tumor and its treatment with conformal radiation therapy (RT) and chemotherapy can disrupt cognitive function. However, the contribution of disease and treatment to long-term cognitive outcome remains to be elucidated. In this study, we performed longitudinal cognitive follow-up in a subgroup of patients who received RT, chemotherapy, or no treatment. Methods Twenty-five LGG patients underwent neuropsychological evaluations at study entry, and 6 and 12 months subsequently; 9 patients had RT +/- chemotherapy prior to enrollment and 16 had no treatment. Results At the initial evaluation, treated patients had impaired performance on motor speed only, but scored 1 standard deviation below normative values on tests of executive functions; untreated patients had no cognitive impairment. Repeated measures analyses of variance showed a significant variation over time (P = 0.03) in nonverbal memory (delayed recall); treated patients' performance improved at the 6-month follow-up to a level comparable to untreated patients, but both groups declined slightly by the 12-month evaluation. In a subset of patients (N = 16) available for an additional cognitive evaluation, significant changes between the 12-month and the long-term follow-up were seen in phonemic verbal fluency, mood and quality of life; untreated patients seen at short intervals improved slightly while treated patients seen at longer intervals declined. Conclusions Longitudinal follow-up showed that both disease duration and treatment with RT +/- chemotherapy contributed to a mild decrement in nonverbal recall and in some aspects of executive functions and quality of life in this group of LGG patients.