RATIONALE: CC16 is an anti-inflammatory protein highly expressed in the airways. CC16 deficiency has been associated with lung function deficits, but its role in asthma has not been established conclusively. OBJECTIVES: To determine 1) the longitudinal association of circulating CC16 with the presence of active asthma from early childhood through adult life and 2) whether CC16 in early childhood predicts the clinical course of childhood asthma into adult life. METHODS: We assessed the association of circulating CC16 and asthma in three population-based birth cohorts: TCRS (years 6-36; N-participants=814, N-observations=3042), BAMSE (years 8-24; Ns=2547, 3438), and MAAS (years 5-18, Ns=745, 1626). Among 233 children who had asthma at the first survey in any of the cohorts, baseline CC16 was also tested for association with persistence of symptoms. MEASUREMENTS AND MAIN RESULTS: 1) After adjusting for covariates, CC16 deficits were associated with increased risk for the presence of asthma in all cohorts (meta-analyzed adjOR per 1-SD CC16 decrease: 1.20[95%CI 1.12-1.28];P<0.0001). The association was particularly strong for asthma with frequent symptoms (meta-analyzed adjRRR: 1.40[1.24-1.57];P<0.0001), was confirmed for both atopic and non-atopic asthma, and was independent of lung function impairment. 2) After adjustment for known predictors of persistent asthma, asthmatic children in the lowest CC16 tertile had a nearly 4-fold increased risk for having frequent symptoms persisting into adult life, compared with asthmatic children in the other two CC16 tertiles (meta-analyzed adjOR: 3.72[1.78-7.76];P<0.0001). CONCLUSIONS: Circulating CC16 deficits are associated with the presence of asthma with frequent symptoms from childhood through mid-adult life and predict the persistence of asthma symptoms into adulthood. These findings support a possible protective role of CC16 in asthma and its potential use for risk stratification.