The study was conducted to investigate the effect of increasing balanced dietary protein for hyper-prolific lactating sows. In total 544 sows (parity 1–4) was allotted to one of six diets from day 2 post-partum until weaning. The diets were analyzed to have a standardized ileal digestible (SID) crude protein (CP) level of 104.3, 113.3, 120.9, 128.5, 139.2 or 150.0 g/kg. At d 2 post-partum litters were standardized to 14 piglets and body weight (BW), back fat (BF) thickness of sows and litter weight were recorded. Body weight, BF thickness and litter weight was also recorded at weaning. On a subsample of 70 sows (parity 2 and 3) milk samples were obtained at day 3, 10 and 17 post-partum and analyzed for fat, CP and lactose. In the analysis of the dose-response data the dietary SID CP concentration were used as explanatory variable. The abovementioned response variables were fitted with linear broken-line, quadratic broken-line and linear regression models. Sow BW and BF loss reached a break point at 143 g SID CP/kg and 127 g SID CP/kg, where sows lost 0.58 kg/d and 3 mm, respectively (P < 0.001). Multiparous sows had a higher average daily gain of the litter than first parity sows (3.07 vs. 2.53 kg/d) at the break point at 135 g SID CP/kg (P < 0.001), but litter size (13.0 ± 1.2 piglets) at weaning was unaffected by dietary treatment (P = 0.30). Milk CP increased to 5.0 g/100 mL until a breakpoint at 136 g SID CP/kg, milk lactose decreased until a breakpoint at 120 g SID CP/kg to 5.3 g/100 mL (P < 0.001) and milk fat increased linearly (P < 0.05). The daily output of milk protein was increased at day 17 until a breakpoint at 130 g SID CP/kg (663–670 g/d; P < 0.001). The content of milk fat increased linearly with increasing dietary SID CP (P < 0.05). There was a tendency towards an increased number of total born piglets in next litter with increased dietary SID CP (P = 0.06), whereas the weaning-to-estrus interval was unaffected by treatment (P = 0.83). In conclusion, increasing dietary SID CP up till 135 g/kg or 850 g SID CP/d increased ADG of the litter, and this increase was caused by increased milk yield and increased daily protein output in milk.