Short-term associations between disease and milk yield were studied in 1594 lactations occurring between 1977 and 1982 in 732 Friesian, Ayrshire and Holstein crossbred cows. Statistically significant differences in milk yield between one week before and one week after clinical diagnosis were observed for ketosis (5.1 kg/d), hypomagnesaemia (4.1 kg/d), mastitis occurring after peak yield (2.1 kg/d) and lameness (1.1 kg/d). Milk yield declined for as much as 2-4 weeks before diagnosis of disease, and total losses in milk yield associated with ketosis or hypomagnesaemia were estimated to be 60-70 kg. No significant differences were found for endometritis or for mastitis when it occurred before peak yield. Milk yield was suppressed for about 4 weeks after calvings with retained placenta, but there were no corresponding effects of dystocia or hypocalcaemia on milk yield. It is suggested that if automatic daily milk recording is available it may be possible to detect deviations from normal, and hence detect subclinical disease 2 or more weeks before its clinical appearance.