Among the different types of households, agricultural ones are perceived as the most exposed to the problem of income instability. Income fluctuations pose a threat not only to the financial stability of farming families but also to the sustainable development of the agriculture sector. Investigating the scale and factors of the variability of farmers’ household incomes is important in designing and monitoring agricultural policy measures that aim to stabilize farmers’ incomes. In this context, the main objective of this article is to assess the income variability of agricultural households in Poland and to identify the main correlates of farmers’ income fluctuations in the period 1996–2022. To emphasize the significance of the problem of income instability for agricultural households, a comparative assessment of income variability was performed (relative to non-agriculture households). Furthermore, two research subperiods (1996–2003; 2004–2022; before and after EU accession) were distinguished to capture the association between the Common Agriculture Policy (CAP) and the stability of agricultural income. The study focusses on associations between variables (regression and correlation analyses were used). The income of agricultural households was shown to exhibit greater variability (year-on-year) than the income of households of workers and of self-employed people, with a lower scale of fluctuations after 2004. Furthermore, the study reveals a group of factors that could be correlated with the variability of the income of agricultural households: income structure (share of income from agriculture), natural and climatic conditions (variability of the yields and agricultural production), and changes in prices of the means of agricultural production. The findings raise, however, a question about the effectiveness of CAP instruments in stabilizing agricultural income.