Using 2007-2009 Medicare Part D data, Zhang et al examined geographic and seasonal variations in antibiotic use. Substantial geographic and seasonal variation existed across regions, after adjusting for population characteristics including demographics, insurance status, and clinical characteristics. These differences could not be explained by differences in the prevalence of the underlying conditions. The South saw the highest antibiotic use, where 21.4% of patients per quarter used an antibiotic, compared with 17.4% in the West (P < .01), the lowest region. Regardless of region, the rate of antibiotic use was highest in the first quarter (20.9% in January-March) and lowest in the third quarter (16.9% July-September) (P < .01).