A temporal cluster of cases may result from exposure to a new carcinogen, but it may also occur under stable conditions or as result of another factor, such as changes in medical practice. In this study, we used several complementary methods to detect a cluster and to conduct an initial investigation regarding its cause. The analyses included three stages, and were based on the time interval between consecutive diagnoses. First, we applied the sets monitoring technique to detect clustering. Following the detection (alarm), a confirmatory analysis was conducted in order to confirm or refute it as a true alarm. We applied this analysis to the first five cases diagnosed subsequent to the alarm. With a graphical display of the temporal pattern of the diagnoses, we considered the possible causes leading to the cluster. The data included registered diagnoses of leukaemia cases made from 1960 to 1990 among residents of Ashkelon, Israel. Clustering was detected for acute lymphatic leukaemia and for chronic myeloid leukaemia. We found that the temporal patterns of the diagnoses consistent with the possibility that these two clusters are due to exposure to a common local carcinogen. We cannot however, rule out the possibility that these clusters are due to unknown factors.