Our objective was to investigate functional outcome in primary lateral sclerosis (PLS). We followed a group of 24 patients with PLS. Clinical (ALSFRS), respiratory, and neurophysiological (electromyography and transcranial magnetic stimulation) evaluations were performed at entry and regularly over the follow-up period. The time taken for a greater than 10% decrease in ALSFRS compared to the first assessment was defined as the dichotomous outcome; prognostic factors were evaluated using the Cox proportional hazard model. Results demonstrated that the median age at symptom onset was 54 years (range 28−75 years). In 46% symptoms began in the lower limbs, in 21% in the upper limbs and in 33% in bulbar muscles. The median symptom duration at first visit was 3.1 years (range 0.9−11.7 years). At last follow-up the median disease duration was 9.9 years (range 4.2−17.6 years). The median follow-up time was 4.6 years (range 2.1−11 years). We excluded from final analysis three patients with positive family history. Older age at onset (p = 0.019) was related to more rapid functional impairment; gender, forced vital capacity, region of onset and neurophysiological changes did not predict outcome. In conclusion, age is the most critical prognostic factor for functional outcome in PLS.