Data from a panel of New York Dairy farms were used to estimate bST adoption functions, and to measure the impact of bST on milk output and profitability per cow. Adoption results are consistent with previous ex ante bST adoption studies. Farm size, productivity, and education of the principal operator are the most important explanatory variables influencing adoption. The use of bST was found to significantly increase milk output per cow net of other explanatory variables, correcting for self-selection with respect to bST use. The impact on profits was, however, not statistically different from zero at any conventional statistical significance level. Copyright 1999, Oxford University Press.