Using data from health records for cows in Finland from between 1983 and 1991, we investigated the general trend for frequency of mastitis in heifers, seasonal variation, breed differences, and etiology of the disease. The performance of the heifers was monitored during first lactation. Treatment of heifer mastitis, defined as mastitis treatment 1 wk before and 1 wk after calving, increased from 1.8 to 4.4% between 1983 and 1991. The increase can be partly attributed to increased milk production, subsequent reduced resistance to mastitis, and the general trend toward more efficient treatment of mastitis. Heifer mastitis was a characteristic disease of well-managed, productive herds with low SCC and a high frequency of mastitis treatments administered by a veterinarian. Of the feeding factors studied, only use of homemade concentrates was associated with higher mastitis frequency. Mastitic heifers had higher genetic potential for milk production than control heifers, but actual milk production was 70 to 80 kg lower than expected. Most mastitic heifers recovered well and were not more susceptible than control heifers to further incidence of mastitis or other diseases. However, heifer mastitis caused more heifers than usual to be culled.