How the interactions among heat stress, calving season and milk yield (MY) affects lactation and body weight (BW) curves was studied by analyzing the daily records of MY and BW of dairy cows. The summer calving cows (SC) attained minimal BW sooner than winter calving cows (WC), the differences between high yielding (HY; MY≥40 l/d) SC and WC being greater than differences between low yielding (LY; MY<40 l/d). The minimal BW was lower in all SC than in WC and this could be related to the differences between the HY cows. MY peak was higher in all WC than in all SC, similar in LY cows among the two treatments, and higher in HY WC than in HY SC. The 4.4% differences in MY during the first 40 wk postpartum between all WC (8960 l) and all SC (8568 l) could be related to 8.0% differences between HY WC (10444 l) and HY SC (9604 l). During the summer peak MY occurred later than during the winter irrespective of the production level. We have shown that heat stress during early lactation affects the entire lactation and BW curves in dairy cows, and that heat stress negatively interacts with MY.