Spurred by previous observations we assessed the relationship between offspring birth weight and parental leukaemia risk in a register-based investigation including 2.4 million parents of 2 million Danish children. Regardless of analytical approach, offspring birth weight was not associated with parental risk of leukaemia overall or of leukaemia subtypes except for a twofold increased acute lymphatic leukaemia risk in fathers of high birth weight offspring and an increasing paternal risk of chronic myeloid leukaemia with increasing offspring birth weight. These may both be chance findings. Our investigation indicates that offspring birth weight is not strongly associated with parental leukaemia risk.