['To prospectively investigate the presence and counts of archaea in feces of 472 children in association with weight development from 6 to 10\xa0years of age.', 'Within the KOALA Birth Cohort Study, a single fecal sample from each child was analyzed by quantitative polymerase chain reaction to quantify archaea (Methanobrevibacter smithii, Methanosphera stadtmanae). Anthropometric outcomes (overweight [body mass index {BMI}\xa0≥\xa085th percentile], age- and sex-standardized BMI, weight, and height z-scores) were repeatedly measured at ages (mean\xa0±\xa0SD) of 6.2\xa0±\xa00.5, 6.8\xa0±\xa00.5, 7.8\xa0±\xa00.5, and 8.8\xa0±\xa00.5\xa0years. Generalized estimating equation was used for statistical analysis while controlling for confounders.', 'Methanobrevibacter smithii colonization was associated with an increased risk of overweight (adjusted odds ratio [OR]\xa0=\xa02.69; 95% confidence interval [CI] 0.96-7.54) from 6 to 10\xa0years of age. Children with high levels (>7 log10 copies/g feces) of this archaeon were at highest risk for overweight (OR\xa0=\xa03.27; 95% CI 1.09-9.83). Moreover, M. smithii colonization was associated with higher weight z-scores (adj. β 0.18; 95% CI 0.00-0.36), but not with height. For BMI z-scores, the interaction (P\xa0=\xa00.008) between M. smithii and age was statistically significant, implying children colonized with M. smithii had increasing BMI z-scores with age.'] Is gut colonization with methanobrevibacter smithii associated with childhood weight development?