Vitamin D plays a role in maintaining calcium and phosphorous homeostasis and is required for normal growth and development of calves. We hypothesized that supplementing dairy calves with 25-hydroxyvitamin D₃ [25(OH)D] would improve development and immunity of dairy calves. The objective of this experiment was to test the effects of dietary 25-hydroxyvitamin D₃ on growth and immune status of dairy heifer calves. One-hundred 43 Holstein heifer calves were assigned to one of 2 treatments, control (CON) and 25-hydroxyvitamin D₃-supplemented (25D), starting at approximately 14 d of age. The control diet was manufactured to provide vitamin D₃ at a rate of 1.5 μg vitamin D₃/kg BW and 25D heifers were supplemented with an additional 1.7 μg 25D/kg BW. The 25D was administered orally. Calves were kept in group pens and fed ad libitum milk replacer and starter grain with an automated feeding system. Treatments were applied daily to individual calves. Bodyweight, whither height, and blood samples were collected weekly from start of treatment until weaning, then every 2 weeks until 16 weeks of age. Data were analyzed with mixed models that included fixed effects of treatment and time and random effects of calf nested within treatment and pen. Serum 25(OH)D concentrations of 25D heifers were greater compared with CON (123 vs. 44 ± 6 ng/mL, P < 0.001). The 25D heifers grew faster compared with CON (ADG = 0.82 vs. 0.78 ± 0.13 kg/d, P = 0.03) such that 25D heifers were 4 kg heavier (P = 0.01) and tended (P = 0.07) to be 1 cm taller at 16 weeks. Milk intake of the 25D heifers was greater than the CON heifers (9 L/d vs 8 L/d, P = 0.03). Intake of starter grain did not differ between treatments. Concentrations of leukocytes in blood, neutrophil phagocytosis and oxidative burst capacity, and concentrations of IgG1 and IgG2 in serum did not differ between treatments. Feeding dairy heifer calves 25-hydroxyvitamin D₃ at 1.7 μg/kg BW plus 1.5 μg vitamin D₃/kg BW increased weight gain compared with just feeding 1.5 μg vitamin D₃/kg BW.