A linear model was used to estimate the effects of left displaced abomasum on 57,383 test day milk yields collected between January 1, 1992 and December 31, 1993 in New York State from 12,572 Holstein cows in parity < or = 6. Twenty-five index categories were created to differentiate among milk collected between calving and up to 60 d after diagnosis of left displaced abomasum, milk collected > 60 d after diagnosis of left displaced abomasum, milk collected before or after diagnosis of cows with any diseases other than left displaced abomasum, and milk collected from cows that remained healthy for the entire lactation. For each parity, the statistical model included fixed effects (management, age at calving, length of dry period, previous 305-d milk yield, stage of lactation, year and season of sampling date, disease index, and DHIA SCS at milk sampling) and random effects (permanent and temporary environments) on test day milk yields. From calving to 60 d after diagnosis, cows with left displaced abomasum yielded on average 557 kg less milk than did cows without left displaced abomasum; 30% of losses occurred before diagnosis. Milk loss increased as parity and productivity increased. Cows with left displaced abomasum were nearly twice as likely to have another disease than were cows without left displaced abomasum.