SummaryData from non-rapid-cycling bipolar (manic-depressive) patients who were receiving long-term treatment with lithium carbonate were analysed by the life table method to determine when lithium carbonate prophylaxis failures occurred. Forty-four of 96 patients failed to keep well in spite of maintenance lithium therapy. The analysis revealed an early, rapid failure rate during the first six months of treatment, which was followed by a slower rate of failure. Several clinical factors were assessed to determine if any of them predicted which patients would experience their initial failure in the early or late interval, but none of these factors, which included age, sex, age of onset, rate of affective attacks, family history, and the nature of the preceding episode, were found to have any predictive value regarding lithium prophylaxis failure. We found, however, that patients who had early failures tended to have a subsequent early failure in spite of continued maintenance with lithium carbonate.