Acetone concentration in milk is correlated with subclinical and clinical ketosis and also with milk yield and reproductive efficiency. The purpose of this study was to assess a threshold of milk acetone at which cows tend to be adversely affected. The critical values were estimated on data from 11690 lactations where the change in daily milk yield from weeks 1 to 2 to weeks 5 to 6 of lactation was evaluated in relation to different milk acetone concentrations. Daily milk yield tended to be reduced in cows with acetone concentrations between 0·7 and 1·4 mmol/l, but responses were not entirely consistent. At concentrations >l·4 mmol/l, daily milk yield was clearly reduced. Long-term milk yield (day 0 to 100 and day 0 to 200) was about 190 kg of 40 g/kg fat-corrected milk (FCM) less in cows with acetone concentrations >l·4 vis-à-vis <0·7 mmol/l. Reproductive efficiency was also impaired in cows with acetone concentrations >l·4 mmol/l; 4·9 days longer interval from calving to first service (first parity and parity 2+ cows) and 5·7 times a greater risk of cystic ovaries (parity 1), compared with cows with milk acetone <0·7 mmol/l. However, no significant effects on long-term milk yield or on reproductive efficiency were found in cows with acetone concentrations 0·7 to 1·4 vis-à-vis <0·7 mmol/l. The results suggest that 1·4 mmol/l acetone in milk may be used as the most important critical value, as higher concentrations are detrimental to productivity. The interval 0·7 to 1·4 mmol acetone per I milk may be used as a warning class, since early lactation yield may be reduced.