The study investigated the determinants of household size in Eastern Uganda. Secondary data from the Uganda National Household Survey (2005/2006) provided by Uganda Bureau of Statistics (UBOS) was used. A total sample of 1922 households in both urban and rural areas of Eastern Uganda was used in the study and their prevailing demographic; socioeconomic; housing and welfare conditions were assessed to establish the significant determinants of household size. The analysis was mainly based on the study questions indicated in the questionnaire attached to the report as an appendix. Data analysis was carried out using SPSS version 10 and the Pearson chi square test and logistic regression model used as analytical tools. The study found out that age, marital status, education level of household head, employment status, dwelling unit and the number of rooms in a dwelling unit, were associated with and significantly determined household size in Eastern Uganda. It was found out that compared to small households, a bigger percentage of respondents from big households were in the 35-44 age interval (31.9% compared to 17.4%). Therefore, it was concluded at this stage that most of the respondents were in their median ages and also, at older ages (75+) more respondents lived in small rather than big households. Household size and age of respondent came outstanding of all other variables and it was clearly confirmed that in age interval 25-34, household members stood higher chances of belonging to small households. It was also found out that unmarried household members were more likely to live in small household sizes than in big households. Most respondents in polygamous marriages belonged to big households. This was related to the number of household members as a result of the many children from different wives. The study, also, revealed that as the level of education of the household head improved, for instance from primary to secondary and beyond, more household heads were found with small household sizes.