This population-based cohort study investigated the occurrence of familial astrocytoma among first-degree relatives of patients with astrocytoma diagnosed between 1985 and 1993 in the northern region of Sweden. The 432 cases received a questionnaire. They were asked to provide names and cancer diagnoses of first-degree relatives. Of the 297 answering, a cohort was constructed of their 1,890 first-degree relatives (FDR). A significantly increased risk [standardized incidence ratio, SIR = 2.12, 95% confidence interval (CI) = 1.18-3.49] was shown for developing primary brain tumors (PBT). In 4.7% (14/297) of the families, a PBT was found. Interestingly, the increased risk was for astrocytoma only (SIR = 3.12, 95% CI 1.42-5.92), and not for other PBT (SIR 0.90, 95% CI 0.18-2.64). When the cohort was divided according to the median age of proband, most of the increased risk was restricted to the younger cohort (SIR = 4.71, 95% CI 1.52-10.99). Surprisingly, a significantly decreased risk for breast cancer and colon cancer was shown. The finding that the increased risk is restricted to astrocytoma only is a novel one. This study implies that familial aggregation of astrocytoma exists; the familial clustering occurs in a small fraction of astrocytoma, and might be explained by inherited factors.