Days open was studied from information in lactation records collected through the National Cooperative Dairy Herd Improvement Program. Information on days open was available for 83% of United States records. Only 33% of records from the Wisconsin dairy records processing center included information on days open because 46% of herds had no reporting of this information, and a computation error caused information on days open for some cows in other herds not to be reported. When records from Wisconsin were excluded, information on days open was available for 90% of lactation records. Four percent of herds had no reporting of information on days open for any cows; 85% had above 80% reporting. Gestations computed from these data matched those from previous studies except for data from the Pennsylvania and Michigan dairy records processing centers, which had longer intervals. Errors in preparing records explained these longer intervals. When records from Michigan and Pennsylvania dairy records processing centers were excluded, gestations were 282 days for Ayrshires, 288 days for Brown Swiss, 286 days for Guernseys, 280 days for Holsteins, 279 days for Jerseys, 282 days for Milking Shorthorns, and 282 days for Red and White. With correction of computing problems and selection of appropriate procedures for missing information, data on days open available through the National Cooperative Dairy Herd Improvement Program appear to provide an adequate basis for adjusting lactation records.