We analyzed the natural history of amyotrophic lateral sclerosis in 277 patients. Our goal was to develop a better understanding of the clinical disease and thus improve the design of therapeutic trials. The Tufts Quantitative Neuromuscular Exam (TQNE) was used as the primary assessment instrument. Our analysis suggested that although more observations are desirable, six monthly TQNEs were adequate to establish the rate of disease progression. We observed a spectrum of deterioration rates without definable subgroups. The striking linearity of deterioration was confirmed. We found a high correlation between deterioration rates in arm and leg strength for individual patients, but a wide range between different patients. Sex and the age at clinical onset did not affect the deterioration rate. As compared with patients without a positive family history, those with other affected family members had a slower loss in arm but not leg strength. We propose that natural history controls can be used effectively in the design of ALS therapeutic trials.