Early detection of clinical mastitis cases is considered key for the success of therapy and to maintain milk quality. Our objectives were to describe variation in 1) clinical mastitis detection frequency among milkers; and, 2) etiology of clinical mastitis among low and high detection milkers. Clinical mastitis records were from 712 cases from one California Central Valley large dairy herd where cows were milked twice a day in 2 separate parlors. Milking procedures included forestripping with observation of milk appearance on the parlor floor. When abnormal milk was observed a milk sample was collected, labeled with the milker ID, and cultured on-farm following the Minnesota Easy Culture system. Culture results were classified as no-growths, gram-negatives, gram-positives, and, mixed culture when gram-negatives and gram-positives were isolated. The total number of cases detected among 10 milkers ranged from 21 to 113 and the number of working hours from 817 to 987. Milkers 1 to 5 detected 2 to 7 cases per 100 h and were classified as low detection milkers. Conversely, milkers 6 to 10 detected 9 to 12 cases per 100 h and were classified as high detection milkers. The percentage of samples from which bacteria were isolated ranged from 38% to 71% among milkers. However, milker’s clinical mastitis detection rate did not affect the percentage of samples with bacterial growth (P = 0.93) - it was 58% for the 2 clinical mastitis detection rate groups. Gram-positives were the bacteria most commonly isolated from both groups with 32% of the samples; gram-negative bacteria represented 21% and 16% of isolates in groups low and high, respectively; and, mixed cultures 5% and 10% in groups low and high, respectively. In conclusion, there was a large variation among milkers in clinical mastitis detection rate and in the percentage of samples with bacterial growth. However, neither the percentage of samples with bacterial growth nor the etiology were influenced by the level of detection. Therefore, it appears that high clinical mastitis detection rates can be achieved without increasing the percentage of cases without bacterial growth.