The aim of this study was to investigate the relationship between measures of body condition score collected from calving until wk 26 of lactation and reproductive measures (calving interval, days to first heat, days to first service, and conception at first service). Since 1973 sires of cows at the Langhill Dairy Cattle Research Centre have been selected for either high (selection line) or average (control line) genetic merit for fat plus protein. The data included 1211 records from 534 cows calving from 1988 to 1999. At first calving, cows were randomly assigned to one of two ad libitum diets: one that was relatively high in concentrates (approximately 3000 kg/yr) and one that was relatively low in concentrates (approximately 1500 kg/yr). Selection line cows were on average thinner and lost more condition in early lactation than control line cows. Cows that lost condition, those that were thinner than average at wk 10 of lactation and those that were thinner on average over the first 10 wk, had poorer reproductive performance. This effect was greatest in the selection line. Line x diet interaction effects were not statistically significant. Genetic correlations between body condition score and reproductive measures were unfavorable and ranged from -0.04 to -0.54. The relationship between body condition score and production was strong, but, even after adjusting for yield, an unfavorable relationship still exists between body condition score and fertility. Body condition score could be used as a management and selection tool to improve reproductive performance.