To document the natural history of von Recklinghausen neurofibromatosis, we followed up a nationwide cohort of 212 affected patients and families identified in Denmark 42 years ago. We obtained follow-up information on 99 percent. Because all 76 probands were identified through hospitals, they may include a disproportionate number of severe cases of neurofibromatosis. To diminish this effect of selection bias, we distinguished between the probands and their affected relatives. In a comparison with the general population, survival rates were significantly impaired in relatives with neurofibromatosis, worse in probands, and worst in female probands. Malignant neoplasms or benign central nervous system tumors occurred in 45 percent of the probands, giving a relative risk of 4.0 (95 percent confidence limits, 2.8 to 5.6) as compared with expected numbers. Multiple primary neoplasms were found in 15 probands, but only 1 relative. Compared with the general population, male relatives with neurofibromatosis had the same rate of neoplasms, whereas female relatives had a nearly twofold higher rate (relative risk, 1.9; 1.1 to 3.1). Nervous system tumors were disproportionately represented. We conclude that patients with severe neurofibromatosis requiring hospitalization often have a poor prognosis, but incidentally diagnosed relatives may have a considerably better outcome.