BACKGROUND : Needle aspiration (NA) is recommended as first-line treatment of primary spontaneous pneumothorax (PSP). We aimed to assess NA success and the effect of a longer symptom onset to NA time. METHODS : A discovery phase was retrospectively conducted in the intensive care unit of Louis Mourier Hospital (January 2000 to December 2011) followed by a prospective validation cohort (January 2012 to August 2015). The primary outcome was immediate NA success defined by the absence of need for chest tube insertion within 24 hours of the procedure. RESULTS : In the discovery phase, 130 patients were admitted for PSP and 98 had NA as first-line treatment (75%). The immediate success rate of NA was 34.7% and was higher when it was performed ≥48 hours after symptom onset (57.7% vs 25%; p=0.004). In the prospective cohort, 87 patients were admitted for PSP; 71 (82%) had NA as first-step treatment. The immediate success rate was 40.8%. NA was more successful when it was performed after 48 hours of symptoms' onset (34.5% vs 7.1%; p=0.005). A delay between the first symptom and NA procedure ≥48 hours was associated with a higher success of NA (OR=13.54; 95% CI 1.37 to 133). A smaller pneumothorax estimated by Light's index was associated with NA success (OR=0.95; 95% CI 0.92 to 0.98). To what extent some of these pneumothoraces would have had a spontaneous resolution remains unknown. CONCLUSIONS : When managing PSP with NA, a longer symptom onset to NA time was associated with NA success. BACKGROUND : NCT02528734.