One of the major radiobiological interests has been to maximize the effectiveness of the time-dose relationship in the clinical setting. Current explorations include altered fractionation schedules, multiple daily fractions and hypofractionation. Patient compliance to standard radiotherapy treatment schedules is taken for granted. To evaluate the true rate of compliance, the charts of all new patients treated from July 1, 1984 through June 30, 1985 were reviewed. The overall incidence of unplanned interruptions was 54% (361/668). The frequency of interruptions is significantly higher in patients treated to the primary site as compared to those treated for metastasis (59.8% and 35.6% respectively). The duration of the interruptions varied: 12.7% of the patients missed only 1 day, 25% missed 2 to 5 days, 38% had interruptions totalling 6-15 days, and in 24% the total exceeded 15 days. The most frequent cause of the unplanned interruptions was a rest resulting from unusually adverse tissue reactions (46.8%-169/361). Although this study has documented that unplanned interruptions are a major problem, the impact on local control and survival cannot be determined from our data. A retrospective review of multi-institutional studies such as those conducted by the Patterns of Care or RTOG might show that one of the major causes of failure is unplanned interruptions.