This paper describes the clinical and laboratory findings from 264 cases of toxic mastitis in cows in Northern Ireland between October 1995 and May 1997. Nearly all the cases occurred during the winter housing period, with 84 per cent occurring between November and March inclusive, and 30 per cent in March. Sixty per cent of the cases occurred within one month of calving, and 29 per cent within four days of calving. The most common clinical signs were lethargy (92 per cent), discoloured milk (90 per cent), anorexia (72 per cent), tachypnoea (23 per cent), diarrhoea (23 per cent), recumbency (18 per cent) and staggering (15 per cent). Severe pyrexia (18 per cent) and clinical dehydration (44 per cent) were relatively common findings. Pure growths of Escherichia coli were isolated from 50 per cent of the milk samples, but 1 1 per cent yielded no bacterial growth. In vitro sensitivity tests indicated that enrofloxacin was effective against 98 per cent of the baderia isolated, and framycetin and amoxycillin/clavulanic acid against 91 per cent. Abnormally high blood urea levels were observed in 31 per cent of cases, high blood creatinine levels in 42 per cent, and severe leucopenia in 56 per cent. Of the cases which were followed up, 14 per cent died, 21 per cent were culled early and a further 22 per cent lost milk production from the affected quarter.