This study investigates delay in diagnosing primary central nervous system lymphoma (PCNSL), which has a variable clinical and radiological presentation. Early diagnosis and treatment may improve survival and cause less sequela in PCNSL. Medical records of all new cases of PCNSL morphologically verified while alive or by autopsy in Norway in 1989-1998 were reviewed (n = 74). The time from initial symptom to final morphological diagnosis of PCNSL had a median (mean, range) of 70 (106, 22-330) days in 16 AIDS patients and 75 (157, 8-1285) days in 58 non-AIDS patients. Among non-AIDS patients, the time to diagnosis was longer in patients with no tumour in the first neuroimaging report after initial symptom (p = 0.001). Median (mean, range) time from initial symptom to neuroimaging was 14 (25, 1-60) days in AIDS patients and 21 (88, 1-1095) days in non-AIDS patients. In the non-AIDS group, those presenting with personality change or visual disturbance had more delayed imaging than the others. The time from first neuroimaging examination to final diagnosis in non-AIDS patients had a median (mean, range) of 28 (69, 1-845) days, and was longer when no tumour was indicated in the imaging report (p = 0.005) and if first biopsy did not confirm the diagnosis (p = 0.02). All AIDS patients had their diagnosis of PCNSL first established by autopsy. The time from first neuroimaging to autopsy had a median (mean, range) of 48 (81, 10-270) days. There is a considerable delay in the diagnosis of PCNSL and strategies for earlier diagnosis are thus needed. Physicians should consider early neuroimaging in patients with personality changes or visual disturbance, early renewed imaging in patients with persistent neurological symptoms but no tumour on initial imaging, and early/repeated biopsy of focal brain lesions in both AIDS patients and non-AIDS patients.