A longitudinal study was conducted over 2 years to identify types of antibiotics and sulfonamides used in Michigan dairy herds for disease prevention and treatment, to determine patterns of use of antibiotics and sulfonamides by herd size and animal age group, and to determine the influence of veterinary presence during diagnosis on the types of antimicrobials used for disease treatments. In order of frequency, the most commonly used preventive antibiotic and sulfonamides were penicillins, tetracyclines, aminoglycosides, and cephalosporins, making up over 86% of all antimicrobials used for disease prevention. The most commonly used therapeutic antibiotics and sulfonamides were penicillins, tetracyclines, aminoglycosides, and sulfonamides, making up over 81% of all antimicrobial drugs used for disease treatment. Cows received the greatest number of drugs, followed by calves (cattle from birth to weaning). Young stock (cattle from weaning to first calving) received the lowest number of drugs. All herds had similar patterns of drug use for the 3 age groups, regardless of herd size. With the exception of polymyxin and chloramphenicol, producers used antibiotics on their own more than with a veterinarian present or on the advice of a veterinarian. Overall, veterinary presence was significantly associated with increased use of tetracyclines, sulfonamides, and nitrofurans, and decreased use of penicillins and aminoglycosides. Implications for drug residue prevention strategies are discussed, with emphasis on the role of the practicing veterinarian.