Three hundred bucket-fed diarrhoeic calves up to the age of 21 days were used to investigate the degree in which d-lactic acid contributes to metabolic acidosis in bucket-fed calves with naturally acquired neonatal diarrhoea. Fifty-five percent of all diarrhoeic calves had serum d-lactate concentrations higher than 3 mmol/l. Mean (±SD) d-lactate values were 5.7 mmol/l (±5.3, median: 4.1 mmol/l). d-lactate values were distributed over the entire range of detected values from 0 to 17.8 mmol/l in calves with base excess of −10 to −25 mmol/l. Serum d-lactate concentration was higher in patients with ruminal acidosis (6.6 ± 5.2 mmol/l; median: 5.9 mmol/l) than in those with physiological rumen pH (5.3 ± 5.4 mmol/l; median: 3.7 mmol/l). There was no evidence of a correlation (r = 0.051) between the serum levels of d-lactate and creatinine (as an indicator of dehydration). d-lactate was correlated significantly with both base excess (r = −0.685) and anion gap (r = 0.647). The proportion of cured patients was not significantly different between the groups with elevated (>3 mmol/l) and normal serum d-lactate concentrations. This study shows that hyper-d-lactataemia occurs frequently in diarrhoeic calves, has no impact on prognosis but may contribute to the clinical picture associated with metabolic acidosis in these animals.