Angelman syndrome (AS) is a profound disorder notable for mental retardation and severe language deficits that results from lack of function of the maternally inherited copy of the UBE3A gene. Chromosome deletions of 15q11q13, paternal uniparental disomy (UPD), UBE3A gene mutations, and imprinting center defects are all commonly recognized mechanisms that disrupt the function of the maternal copy of the UBE3A gene. We report here two patients with different atypical etiologies of AS. The first patient is a 3-year-old boy with global developmental delay, severe speech deficits, seizures, and very happy disposition. Southern blot analysis for the maternal and paternal chromosome 15 methylation products showed a mosaic methylation pattern, suggesting an imprinting center defect. The second patient is a 4(1/2)-year-old boy with global developmental delay, no expressive language, microcephaly, seizures, and ataxic gait. Array-based comparative genomic hybridization (CGH) demonstrated a loss in copy number for two overlapping clones encompassing the UBE3A gene, indicating a partial deletion within UBE3A. His mother, who was adopted, had an identical pattern, suggesting that her deletion was probably on her paternally imprinted allele. These patients illustrate the expanding spectrum of molecular findings in AS, reinforce the need to maintain suspicion when clinical features suggest AS but initial testing is normal, and show the power of CGH as a tool to uncover partial UBE3A deletions.