This study examined lactation curve parameters and their genetic relationships in Holstein cows using 1,684 test-day milk yield records collected from 2017 to 2019 in Fayoum Governorate, Egypt. Wood's gamma function modeled lactation curves. Overall means for test-day milk yield (TDMY), initial yield (a), ascending phase (b), descending phase (c), persistency (P), days to peak yield (PY), and maximum yield (Ymax) were 25.98 kg, 12.72 kg, 0.401 kg, 0.008 kg, 6.76 kg, 50.12 days, and 29.43 kg, respectively. Heritability estimates ranged from 0.14 (persistency) to 0.36 (ascending phase), while TDMY, initial yield, and maximum yield showed moderate heritability's values (0.26, 0.23, and 0.29, respectively). Strong positive genetic correlations were found between TDMY and Ymax (0.97), and TDMY and persistency (0.79). Initial yield showed negative genetic correlations with ascending slope (-0.39) and days to peak milk (-0.64). The rate of milk yield increase (b) negatively correlated with persistency. Days to peak milk (PY) positively correlated with ascending slope (0.77) and negatively with descending slope (-0.55). These findings provide insights into the genetic control of lactation curve parameters in Holstein cows, offering potential for genetic improvement through selective breeding and developing effective strategies to enhance milk production efficiency and persistency in Egyptian dairy herds.