Technical Abstract: Factors that affect frequency of death were investigated for lactating cows in 45,032 herds for lactations from 1995 through 2005. Analyses included effects of herd, year, month, and stage of lactation in which lactation ended, parity, breed, and milk yield. A sample of 1,645 herds was employed to compare two methods of analysis (PROC GENMOD and PROC GLM). Because the two methods ranked effects similarly, the large national sample (15,025,035 lactations) was analyzed with PROC GLM. Overall frequency of death per lactation was 3.1% (5.7% per cow). Frequency of death increased by 1.6% from 1995 to 2005. Frequency suddenly increased by 0.9% from 2003 to 2004, probably due to the requirement for euthanizing downer cows. Frequency of death was 16.5% higher for lactations terminating < 46d than for those terminating > 250d. Death rate increased with parity (2% higher for 8th parity compared to first parity). Deaths were most frequent in July, least frequent in November. Within-herd breed differences (Holstein, Jersey, Other) were small. Frequency of death increased as lactation milk yield increased. Heritability estimate of likelihood of death estimated from a sample of 79,162 Holstein cows was 1.3%.