PARC/CCL-18 is a protein with unknown function, secreted predominantly by lung macrophages. Sin et al analyzed the relationship of PARC/CCL-18 to COPD using two large preexisting cohorts, the Lung Health Study and ECLIPSE, which they used as derivation and replication cohorts, respectively. PARC/CCL-18′s associations with outcomes were inconsistent across the cohorts. In ECLIPSE, the protein was elevated in people with COPD and was independently associated with COPD and total mortality; in LHS, it was associated with low FEV1 and ‘cardiovascular hospitalization or mortality’. AJRCCM 2011;183:1187-1192.