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Objectives: We aimed to determine the incidence of periodic and rhythmic patterns (PRP), assess the interrater agreement between electroencephalographers scoring PRP using standardized terminology, and analyze associations between PRP and electrographic seizures (ES) in critically ill children. Methods: This was a prospective observational study of consecutive critically ill children undergoing continuous electroencephalographic monitoring (CEEG). PRP were identified by one electroencephalographer, and then two pediatric electroencephalographers independently scored the first 1‐h epoch that contained PRP using standardized terminology. We determined the incidence of PRPs, evaluated interrater agreement between electroencephalographers scoring PRP, and evaluated associations between PRP and ES. Results: One thousand three hundred ninety‐nine patients underwent CEEG. ES occurred in 345 (25%) subjects. PRP, ES + PRP, and ictal–interictal continuum (IIC) patterns occurred in 142 (10%), 81 (6%), and 93 (7%) subjects, respectively. The most common PRP were generalized periodic discharges (GPD; 43, 30%), lateralized periodic discharges (LPD; 34, 24%), generalized rhythmic delta activity (GRDA; 34, 24%), bilateral independent periodic discharges (BIPD; 14, 10%), and lateralized rhythmic delta activity (LRDA; 11, 8%). ES risk varied by PRP type (p <.01). ES occurrence was associated with GPD (odds ratio [OR] = 6.35, p <.01), LPD (OR = 10.45, p <.01), BIPD (OR = 6.77, p <.01), and LRDA (OR = 6.58, p <.01). Some modifying features increased the risk of ES for each of those PRP. GRDA was not significantly associated with ES (OR = 1.34, p =.44). Each of the IIC patterns was associated with ES (OR = 6.83–8.81, p <.01). ES and PRP occurred within 6 h (before or after) in 45 (56%) subjects. Significance: PRP occurred in 10% of critically ill children who underwent CEEG. The most common patterns were GPD, LPD, GRDA, BIPD, and LRDA. The GPD, LPD, BIPD, LRDA, and IIC patterns were associated with ES. GRDA was not associated with ES. [ABSTRACT FROM AUTHOR]