Data from 4143 Holstein cattle from three herds in Turkey were used to study the influences of genetic and environmental factors on milk production traits as well as genetic and environmental trends over 20 years from 1987 to 2006. First lactation mean values for 305-day milk yield, actual lactation milk yield, and lactation length were 6222 +/- 35.8 kg, 6651 +/- 42.6 kg, and 327 +/- 1.3 days. Second and third 305-day lactation yields were proportionately 8% and 11% greater, respectively. The effects of herd and calving year were the largest sources of variation for all traits. Heritabilities of actual lactation milk yield, 305-day milk yield, and lactation length for first lactation records were 0.24 +/- 0.007, 0.23 +/- 0.007, and 0.08 +/- 0.035. Estimates of variances and genetic parameters were consistent with estimates reported throughout the literature. Trends were inconsistent with some phenotypic increase in yields during the period between 2001 and 2005. This improvement was associated with increased environment parameter estimates during that period. No consistent genetic improvement was observed.