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To investigate the natural history and prognostic factors in patients with nonhereditary, adult-onset progressive muscular atrophy.Inception cohort conducted for 18 months.Three university hospitals in the Netherlands (referral centers for neuromuscular diseases).Thirty-seven consecutive patients newly diagnosed (onset of weakness <4 years) with progressive muscular atrophy enrolled between 1998 and 2001.Disease progression was measured at 0, 3, 6, 9, 12, 15, and 18 months by the Medical Research Council sum score, number of affected limb regions, and the Amyotrophic Lateral Sclerosis Functional Rating Scale score. Multivariate linear regression analysis was used to identify predictors of poor outcome. Clinical features and classification of phenotype during follow-up were evaluated. Survival analysis was planned after data collection, performed 5 years after the end of the study.Significant decline of muscle strength (mean, 6.01 Medical Research Council sum score points [95% confidence interval [CI], 3.84–8.18]; P value <.001) and significant increase in the number of affected regions (mean, 0.53 affected region [95% CI, 0.42–0.65]; P value <.001) and functional impairment (mean, 1.85 Amyotrophic Lateral Sclerosis Functional Rating Scale score points [95% CI, 1.38–2.33]; P value <.001) were found. Vital capacity (VC) at baseline and decrease of VC during the first 6 months were significantly associated with outcome. Median survival duration after initial weakness was 56 months.This study shows that patients with progressive muscular atrophy have a relentlessly progressive disease course. Patients with a low VC at baseline and a sharp decline of VC during the first 6 months have an especially poor prognosis.