An epidemiological study of risk factors for involuntary extended lactations (15 to 40 mo) using a multiple variable logistic regression was carried out on 3278 high-yielding dairy cows in an intensive well-managed Hostein herd, milked 3 times daily in northern Mexico. Additional objectives were to assess the association of lactation length (15 to 40 mo) with milk yield and to assess the effect of multiple services (4 to ≥14) on pregnancy per artificial insemination. Also, a survival analysis was performed using the Cox proportional hazards model to test how the occurrence of pregnancy affects lactation length. Retained placenta (odds ratio (OR) = 1.3), metritis (OR = 1.8), ketosis (OR = 1.4), peak milk yield (<50 vs > 50 kg, OR = 1.4), temperature-humidity index at 60 d postpartum (<82 vs > 82 units, OR = 1.4), and 305-d milk yield (<11,000 vs > 11,000 kg, OR = 1.6) significantly increased the risk for lactations > 15 mo. Primiparous cows had less than half the risk of extended lactations (OR = 0.3) compared with multiparous cows. Once a cow had conceived, her risk of having a prolonged lactation dropped sharply (P < 0.01). A strong linear association was found between lactation length and total milk yield for primiparous (450 to 1349 d in milk, maximum milk yield 37,852 kg; r = 0.71) and pluriparous (450 to 1221 d in milk, maximum milk yield 38,021 kg; r = 0.75). Pregnancy per artificial insemination (P/AI) in cows with extended lactation decreased linearly as number of services increased (P/AI = 50.5% for 4 services and 12.8% for ≥14 services). The data showed that well-managed Holstein cows milked 3 times daily were capable of lactating up to 40 mo with remarkable high persistency and with high milk yield at dry-off. Additionally, this study supports previous findings indicating that reproductive and metabolic disorders associated with calving are important risk factors for extended lactations, derived from the link of periparturient diseases withdepressed reproduction in dairy cows.