Background: Poor growth and breakfast eating patterns are associated with delayed mental development and that there is a relationship between impaired growth status and both poor performance and reduced intellectual achievements. The objective of this study was to assess the effects of breakfast eating patterns and anthropometric measurements on cognitive function among early adolescents in the Rural Sidama, Southern Ethiopia. A cross-sectional study was conducted from June to July 2012. Structured questionnaire was used to capture breakfast eating patterns, socio-economic and demographic factors. Anthropometric status was measured using the UNICEF SECA weighing scale and shorr measuring board. Kaufman Assessment Battery for Children-II tests was used for cognitive function measurement. A representative sample size of 211 participants was selected randomly from 4 kebeles. The data was analyzed with SPSS version 16.0 software and WHO anthroplus version 1.04. Results: Of the 208 interviewed, 52% were girls while 48% were boys with mean (±SD) age of 12.01±0.82 years. Breakfast skipping prevalence was 42.3%. Breakfast eating patterns and height-for-age Z score were significant predictors of Pattern Reasoning cognitive test scores (P<0.001). Body mass index for age Z score was a significant predictor (P<0.001) of a combined Simultaneous scale. Regular breakfast pattern, height for age and body mass index for age Z score were significantly (P<0.001) associated with Pattern Reasoning explaining 28.8% variation. Conclusion: Adolescents who were stunted and underweight had lower cognitive test scores compared to those who were normal as well as those who consume breakfast irregularly. Anthropometric status and breakfast eating patterns was significant predictor of cognitive function of adolescents in the study area. We recommended that, parents and adolescents should be educated and trained on healthy breakfast eating patterns and good nutrition practices for healthy cognitive development.