The association of birth type (single vs. double) and pregnancy type (single vs. double) with lactation per-formance and lactation curve was investigated in Holstein cows in Iran. Data of 243298 lactations on 138021 cows in 261 herds collected during January 2000 to December 2012 were used. The average twin-ning rate was 3. 64% and ranged from 1. 44% (primiparous) to 5. 41% (multiparous). Factors associated with twining were calving season, calving year, herd, and parity. The rate of twinning increased from 2. 49% in 2000 to 3. 42% in 2012. The probability of twinning was higher in multiparous cows than that in primiparous [Odds ratio (OR) (95% confidence interval (CI))= 3. 64 (3. 44-3. 84) for multiparous vs. primiparous cows]. The probability of twin pregnancy was higher in cows previously delivering twins than that in those delivering singles [OR (95% CI)= 3. 17 (3. 02-3. 34) for cows delivering twins vs. cows delivering singles]. The lacta-tion curve for cows delivering singles in previous gestation and pregnant with twins in subsequent gestation tended to be lower and flatter. The incidence of twin pregnancy was increased as the cow reached its peak yield earlier, and produced more milk at peak lactation. Primiparous cows delivering singletons in their previous gestation and having twins in their subsequent gestation produced more 100-d, 200-d and 305-d milk than those pregnant with singletons. Multiparous cows that delivered singleton in previous gestation and were pregnant with twin in the subsequent gestation, produced more milk during the first 100-d and 200-d of lactation than those pregnant with singles.