Two experiments were carried out simultaneously in a temperate environment (T), at Sourches (west France), with 24 Landrace X Large White sows and in a tropical environment (TRO), in Guadeloupe, France (16° latitude N, 61° longitude W), with 18 Large White sows, to study the effect of incorporating fat into the lactation diet on sow and litter performance. In each environment, multiparous sows were divided into three groups and given either a control diet (C) containing 20 g fat per kg, or the same diet enriched with a 50 : 50 peanut-rapeseed oil mixture, so as to obtain a fat content of 80 g/kg (medium fat, MF) or 140 g/kg (high fat, HF). A restricted experimental diet was supplied to the sows from day 105 of gestation until farrowing and then ad libitum throughout the 28-day lactation period. The piglets all had ad libitum access to solid food from day 21 to day 70. Milk samples were collected 24 h post partum (colostrum) and at day 21 of lactation. Chemical composition of milk was determined and fatty acid composition was assessed by capillary gas chromatography. Compared with T, the TRO sows showed an increase (P < 0⋅001) in rectal temperature and respiration rate but a decrease in food intake (P < 0·01), milk yield (P < 0⋅001) and litter weight gain from birth to weaning (P < 0⋅05) as well as from weaning to day 70, reaching 25 to 30 kg live weight (P < 0⋅05). However, as dietary fat level increased, the T sows showed a linear decrease (P < 0⋅01) in food intake and no significant change in metabolizable energy (ME) intake during the lactation period. In TRO, the MF diet led to a 0⋅22 proportional increase (P < 0⋅01) in ME intake through a change in nycthemeral feeding behaviour. Both environment and increased dietary fat level significantly affected the fat level and fatty acid composition of colostrum as well as mature milk, particularly n-3 polyunsaturated fatty acid concentration in milk. In conclusion, the addition of fat to the lactating sow diet may be more beneficial under high ambient temperatures than in thermoneutral conditions.