This thesis discusses the results of an observational study conducted in dairy herds in the south-western Sweden and a questionnaire survey of dairy herds done throughout Sweden. In the observational study the health of 3 081 heifer calves in 122 dairy herds was monitored from birth until 90 days of age. Disease incidence was recorded by farmers and by project veterinarians. The calves’ heart girth was measured at birth and at weaning. The average growth of the calves was 600 g per day. We investigated the effects of calf-level, herd-related and dam-related factors on growth and the incidence risk of infectious diseases. The total morbidity found was 23%; enteritis and respiratory disease were the most common diagnoses. The placing of calf pens along an outer wall, ingestion of first colostrum by suckling, receiving colostrum from a younger cow, being of Swedish Red and White breed (SRB) and birth during the summer were significantly associated with an increased risk for enteritis. A low ammonia concentration, draught, bovine viral diarrhoea virus infection in the herd, a poor capacity of the farmer to keep accurate records, being a cross-breed, housing in large group pens, and birth during the winter season, during the night, in a cubicle, a group calving pen or at pasture, as well as unsupervised calving, and factors related to the health of the dam were significantly associated with an increased risk of respiratory morbidity. Receiving colostrum from a young cow, absence of draught, being housed in a calf pen placed along an outer wall and being born during the night were found to be significantly associated with an increased risk of other infectious diseases. Growth was negatively affected by being of SRB breed, housing in large group pens, disease, difficult calving, first parity of the dam and retained placenta. The questionnaire was sent to 1 500 dairy farmers in Sweden and asked for routines from birth to first calving for replacement heifers. It identifies several areas in which advisory input is urgently needed, such as the colostrum routines, the heating of whole milk and the housing of calves and heifers
