Sporadic abortions are inevitable in all herds. However, once the incidence rate exceeds 3 to 5 per cent, particularly when several cases occur close together (i.e. an abortion outbreak), efforts should be made to determine the cause, so appropriate control measures can be taken. In many cases, determining the cause of abortions can be rather difficult particularly when investigating a single abortion. There is a better chance of finding the cause when an abortion outbreak is investigated but a systematic approach must be adopted. This involves taking samples from the aborted foetus, the placenta as well as the dam. For several years now, Neospora is the most commonly diagnosed cause of abortion in New Zealand. Abortions due to Neospora infection usually occur in mid-gestation (between April and June for spring calving herds) and often involve younger cows. There is an increased chance of these cows aborting again the following year. Fungal abortions associated with the feeding of mouldy hay or silage tends to occur later in gestation (May to July). In some cases, the animal may develop a fungal pneumonia, which is invariably fatal. There is little point in examining the suspect feed material as the fungi involved are normal inhabitants and will almost always certainly be present. The needles of Macrocarpa, Leyland cypress and Pinus radiata trees can cause abortion and pregnant cows should be kept away from paddocks close to these trees. Other causes of abortion include viruses (e.g. BVD-virus infection), genetic defects, nitrate poisoning and possibly trace element deficiencies.