Primary spontaneous pneumothorax (PSP) is not an uncommon disease, especially in patients with risk factors such as male gender, history of smoking, and low body mass index (BMI). Amyotrophic lateral sclerosis (ALS) is a rare disease caused by neurodegeneration of the motor neurons that share risk factors with PSP. The aim of this study was to determine the prevalence of PSP in ALS and find the significant risk factors related to PSP. We retrospectively reviewed the data from 86 patients with clinically probable or definite ALS from three different centers. Clinical characteristics, including age, sex, subtype, disease duration, body mass index, history of smoking, tracheostomy state, and ventilator use, were obtained. The ALS Functional Rating Scale-Revised Form (ALSFRS-R) total score and subscores were also retrieved from the medical records. In the results, six of the 86 patients (7%) had PSP. There were no statistically significant differences among the clinical characteristics and the ALSFRS-R scores between the patients with and without PSP, except for BMI and smoking (p < 0.022 and p < 0.019, respectively). A multivariate logistic regression analysis of smoking and BMI showed an odds ratio of 19.25. In conclusion, the existence of PSP in ALS may be under-recognized. Further well-designed, large studies are needed to elucidate the prevalence and pathophysiology of pneumothorax in ALS. For more information click here.