BACKGROUND Amyotrophic lateral sclerosis (ALS) affects people of all ages, but whether the wide range of age at onset is due to distinct diseases or merely reflects phenotypic variability of the same disorder is still unknown. The purpose of this study is to describe clinical and prognostic features of young-adult ALS, with onset before age 40 years, and to compare them with features of the common adult-onset type. METHODS We analyzed clinical features and long-term follow-up of 57 young-adult ALS patients, with disease onset between 20 and 40 years, and compared them with 450 patients affected by adult-onset ALS. RESULTS We found that the majority of young-adult patients showed a predominant upper motor neuron (p-UMN) ALS, characterized by marked spastic paraparesis, with lower motor neuron signs confined to the upper limbs. The proportion of patients with p-UMN ALS phenotype was 59.6% in the young-adult patients and 17.4% in the adult-onset form (p < 0.0001). Young-adult ALS with p-UMN phenotype had longer survival than did the classic phenotype: median survival was 74 months (range 10-226, 95% CI 60.61-87.38) in the former and 56 months (range 6-106, 95% CI 48.65-63.34) in the latter (p = 0.03). In the young-adult patients, a marked male excess was observed in the p-UMN ALS group (5.8:1), whereas the ratio of men to women was 1.1:1 in the classic phenotype (p = 0.01). CONCLUSIONS Our findings show that young-adult amyotrophic lateral sclerosis with the predominant upper motor neuron phenotype represents a distinctive clinical variant characterized by a unique clinical pattern, longer survival, and male prevalence.