The goal of this randomized clinical trial is to find out whether a conservative management strategy (active observation without an initial invasive procedure) can treat as effectively as the standard treatment with chest tube drainage in adults with a large primary spontaneous pneumothorax who are clinically stable. Participants include men and women 18 to 60 years old with a lung collapse ≥30% (by Collins method). The main questions are: * Is conservative management not inferior to chest tube drainage for achieving complete lung re-expansion at 8 weeks (on chest X-ray)? * What are the differences between the two strategies in complications, recurrence at 2 years, quality of life, length of hospital stay, number of additional procedures, and healthcare costs? Comparison groups: * Conservative management: in-hospital observation with a control chest X-ray no earlier than 4 hours; if the person remains stable, discharge home with scheduled outpatient follow-up and serial X-rays until resolution. * Standard treatment (chest tube): chest tube placement and hospital admission; discharge based on clinical progress with routine outpatient follow-up. Researchers will compare both groups to see whether conservative management works just as well to resolve the pneumothorax at 8 weeks and whether it leads to fewer complications, shorter hospital stays, better quality of life, and lower costs. What will participants be asked to do? Review the participant information sheet and sign informed consent. Be randomized by computer (REDCap) to either conservative management or chest tube drainage. Depending on the assigned group: * Conservative group: remain under observation initially, have a chest X-ray at ≥4 hours; if still stable, go home with clinic visits (about 1 week after discharge and then every \~2 weeks) until the pneumothorax has resolved. * Chest tube group: undergo chest tube insertion, stay in the hospital, and attend routine outpatient follow-up after discharge. Undergo chest X-rays interpreted by the clinical team (with centralized review procedures). Complete brief symptom assessments, quality-of-life questionnaires (e.g., EQ-5D-5L), and attend follow-up visits or phone calls for up to 2 years to check for recurrences. This is a multicenter study conducted in several hospitals in Spain. The planned sample size is 436 participants (218 per group), with a non-inferiority margin of 5% and the primary assessment at 8 weeks.
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Complete lung re-expansion on chest X-ray
Timeframe: 8 weeks after randomization.