Background With the improvement of life expectancy, the world faces increasing demands for care of older persons. In this manuscript, we define the characteristics of primary informal caregivers (PIC) of patients aged 75 years and older admitted to geriatric day hospitals (GDH) in Belgium. A PIC is defined as the person who most often provides care and assistance to persons who need to be cared for. We describe PIC socio-demographic characteristics, satisfaction, burden and wishes about caring; the type of assistance provided and received, their self-rated health, socio-demographic and medical characteristics of proxies, in particular the presence of behavioural disorders. Methods We conducted a cross-sectional study in 25 GDH. Participants Four hundred seventy-five PIC of patients ≥75 years and their proxies. PIC completed a questionnaire at the GDH assessing burden by Zarit Burden Index-12 (ZBI-12), self-rated health, social restriction due to caregiving and financial participation. We compared the characteristics of PIC with high and low burden, and the characteristics of spouses and adult children PIC. We also analyzed factors associated with a high burden in a multivariable logistic regression model. Results PIC were mainly women (72%), adult children (53.8%) and spouses (30.6%). The mean age was 64 ± 14 years for PIC and 84 ± 5 years for care recipients. PIC helped for most of Activities in Daily Living (ADL) and Instrumental ADL (iADL). The median ZBI-12 score was 10 [IQR 5–18]. In multivariable regression analysis, a high burden was positively associated in the total group with living with the relative (p = 0.045), the difficulty to take leisure time or vacation (p <  0.001), behavioral and mood disorders (p <  0.001;p = 0.005), and was negatively associated with bathing the relative (p = 0.017) and a better subjective health status estimation (p <  0.001). Conclusion Primary informal caregivers, who were predominantly women, were involved in care for ADL and iADL. A high burden was associated with living with the relative, the difficulty to take leisure time or vacation and the relative’s behavioral and mood disorders. Bathing the relative and a subjective health status estimated as good as or better than people the same age, were protective factors against a high burden.