OBJECTIVE: We studied the evolution of lower respiratory symptoms at 1 month (initial) and 19 months (follow-up) after the collapse of the World Trade Center on September 11, 2001 (9/11). METHODS: A total of 1588 New York police officers completed initial self-administered questionnaires. The level of 9/11 exposure and pre-9/11 health was available in 1373. Of those, 471 (426 with no pre-9/11 chronic respiratory disease) completed a follow-up telephone survey. RESULTS: Prevalence of cough was 43.5% at both initial and follow-up assessments, but increased were the prevalence of phlegm (14.4% to 30.7%, P<0.001), shortness of breath (18.9% to 43.6%, P<0.001), and wheeze (13.1% to 25.9%, P<0.001). Rates of delayed-onset (present on follow-up assessment only) cough, phlegm, shortness of breath, and wheeze were 21%, 21.9%, 31.7%, and 17.3%, respectively. CONCLUSIONS: Most of the lower respiratory symptoms increased between 1 month and 19 months after 9/11.