BACKGROUND. Little is known about the clinical presentation and epidemiology of influenza A H1N1pdm in children in developing countries. We assessed the severity of influenza A H1N1pdm in children in Nicaragua by comparing H1N1pdm cases to seasonal influenza cases in an ongoing cohort study. METHODS. The Nicaraguan Influenza Cohort Study was established in June 2007 to study the burden and seasonality of pediatric influenza in a tropical developing country. During the period from June 2007 through November 2009, a total of 4391 children aged 2-14 years participated in the cohort. We examined the attack rate of clinical influenza and assessed symptoms at first presentation in febrile patients with H1N1pdm versus those with seasonal influenza A or B. RESULTS. The estimated clinical attack rate of H1N1pdm in the cohort was 20.1%, compared to 11.7% and 15.1% for seasonal influenza A and 11.9% and 24.2% for seasonal influenza A and B in 2007 and 2008, respectively. Symptoms significantly associated with H1N1pdm cases versus seasonal influenza A cases were sore throat (adjusted odds ratio [OR], 1.7; 95% confidence interval [CI], 1.2-2.5), wheezing (OR, 5.1; 95% CI, 1.3-19.0), rhonchi (OR, 4.6; 95% CI, 1.4-15.0), crepitations (OR, 16.2; 95% CI, 2.1-128.7), pneumonia (OR, 8.0; 95% CI, 1.7-37.3), nausea (OR, 2.8; 95% CI, 1.5-5.1), and loss of appetite (OR, 2.1; 95% CI, 1.4-3.1). In addition, 3 concurrent influenza and dengue virus coinfections were identified. CONCLUSIONS. Children with influenza A H1N1pdm presented with significantly more symptoms of lower respiratory infection and gastrointestinal symptoms than children with seasonal influenza. The clinical influenza attack rate was high in both pandemic and seasonal years.