From 1975 until 1987, 122 patients without (Group I, mean age 34 yrs) and 20 with pre-existing lung disease (Group II, mean age 66 yrs) were treated for their first spontaneous pneumothorax. Mean follow-up was 5.2 yrs (max. 12.8 yrs). There were 32 chest X-ray verified recurrences, 72% occurring during the first 2 yrs. One, 5 and 10 yr cumulative recurrence freedoms +/- standard error in groups I and II combined were 85 +/- 3%, 75 +/- 4% and 69 +/- 5%, respectively. Using Cox regression analysis in groups I and II combined, pulmonary fibrosis, age greater than or equal to 60 yrs, and height/weight ratio were independent predictors of recurrence. Combinations of these risk factors identified four risk strata with 10 yr recurrence freedoms ranging from 83-0%. Independent predictors in group I alone were pulmonary fibrosis, age greater than or equal to 60 yrs, height/weight ratio, and nonsmoking, resulting in four risk strata with 10 yr recurrence freedoms ranging from 98-31%. Recurrences after first-time treated spontaneous pneumothorax showed a distinct time-related pattern and should accordingly be analysed as a time-related event. In both the total patient-population and in the group without pre-existing lung disease, independent predictors of recurrence were identified, which allowed the patients to be substratified into groups with widely different recurrence rates.