Objective To investigate whether hospital readmission after admission for heart failure (HF), myocardial infarction (MI), and pneumonia varies by season. Data Sources All patients in 2005–2009 Healthcare Cost and Utilization Project State Inpatient Databases for New York and California hospitalized for HF, MI, or pneumonia. Study Design The relationship between discharge season and unplanned readmission within 30 days was evaluated using multivariate modified Poisson regression. Principal Findings Cohorts included 869,512 patients with HF, 448,945 patients with MI, and 813,593 patients with pneumonia. While admissions varied widely by season, readmission rates only ranged from 25.0 percent (spring) to 25.6 percent (winter) for HF (p > .05), 18.9 percent (summer) to 20.0 percent (winter) for MI (p < .001), and 19.4 percent (spring) to 20.3 percent (summer) for pneumonia (p < .001). In adjusted models, in New York, there was lower readmission in spring and fall (RR: 0.98, 95% CI: 0.96–0.99 for both) after admission for HF and higher readmission in spring (RR: 1.04, 95% CI: 1.01–1.07) after MI. In California, there was lower readmission in spring and winter (RR: 0.95, 95% CI: 0.93–0.96 and RR: 0.96, 95% CI: 0.94–0.98, respectively) after pneumonia. Conclusions Given marked seasonality in incidence and mortality of HF, MI, and pneumonia, the modest seasonality in readmissions suggests that readmissions may be more related to non–seasonally dependent factors than to the seasonal nature of these diseases.