About 2,000 patients with primary CNS cancers, as defined by the ICD-O, were registered in four adjacent cancer registries in South and East Netherlands in 1989-1994, covering a population of more than 5 million people. About 85% of the patients were registered through pathological laboratories, concentrated in four centers for neurosurgery. Patients with a clinical-radiological diagnosis only (about 50% in the age-group of 75 years and older) were mainly identified through medical records of hospitalized patients. World-standardized incidence rates of 6.5 and 4.4 per 100,000 person-years for males and females, respectively, were similar to those reported in other European cancer registries. Eighty-five percent of histologically verified primary CNS cancers were of glial origin, 6% were lymphomas and 4% embryonal tumors. Gliomas were mainly of astrocytic (about 85%) and oligodendroglial or mixed type (about 10%). Without additional review most astrocytic tumors could be classified into high-grade (70-75%) and low-grade astrocytomas (20-25%). Different grading systems were used for gliomas, but the dichotomy of astrocytomas according to differentiation grade seemed to correspond well with the definition of the WHO. Age-specific incidence rates for low-grade astrocytomas were remarkably constant. The incidence of high-grade astrocytomas increased sharply with age and declined after the age of 70, whereas the incidence of clinically diagnosed tumors continued to increase. Male/females-ratios were relatively high for these tumor types (1. 6-1.7). We conclude that the registration of primary CNS cancers in the Netherlands may be almost complete and valid for gliomas, embryonal tumors and lymphomas.