BACKGROUND: Singapore is a tropical country with a temperature range of 23 degrees C to 35 degrees C and relative humidity of 48% to 100% throughout the year. Influenza incidence peaks in June through July and November through January, though influenza cases can be detected throughout the year. METHODS: Between May 1 and July 28, 2009, a novel dual-gene diagnostic polymerase chain reaction assay targeting the hemagglutinin (HA) and nucleoprotein (NP) genes of the new influenza A(H1N1/2009) virus was specifically designed for enhanced influenza surveillance using nasopharyngeal swabs collected from symptomatic patients (including their close contacts) and returning travelers returning from influenza A(H1N1/2009)-affected areas, presenting to affiliated primary care clinics as well as the main hospital emergency department. RESULTS: From the week of June 16 to June 23, 2009, this pandemic influenza A(H1N1/2009) displaced and then replaced the seasonal influenzas (H3N2, H1N1, and B). Of 2683 samples tested during this 12-week surveillance period, 742 (27.6%) were positive for any influenza virus using this assay, with 547 cases of A(H1N1/2009) (20.4%), 167 cases of A(H3N2) (6.2%), 14 cases of A(H1N1) (0.5%), and 12 cases of influenza B (0.4%). Results of multivariate analysis showed that age (P < .001), fever (P < .001), cough (P < .001), sore throat (P = .002), rhinorrhea (P = .001), and dyspnea (P < .001) were significantly different among these groups. CONCLUSIONS: From this large prospective study, there was a lower incidence of fever and dyspnea in patients with pandemic influenza A(H1N1/2009) infection. Similar to reports from elsewhere, it was also found that this pandemic virus tends to infect younger people, though with fewer symptoms, on average, than seasonal influenza. Early pandemic influenza A(H1N1/2009) infections appeared to be slightly milder than seasonal influenza as indicated by different symptom patterns in the presentation of more than 500 cases of influenza A(H1N1/2009) during April through July to a large teaching hospital in Singapore.