An observational prospective study of an quantitative approach, aimed at analyzing the workload of nurses, according to the hours of care required by patients, and the allocation of the nursing staff, per patient a day, and its relationship with the occurrence of adverse events and incidents, in four intensive care units (ICUs) belonging to a university hospital, of a tertiary level, in São Paulo, Brazil. This research was performed in the ICU on the 4th floor (Emergency Room and Hematology) and the 6th floor (Pulmonary and Clinics). The sample consisted of 86 patients. To measure the hours of care required by patients, it was applied in a 40-day study, Nursing Activities Score. The data from this study was obtained from the daily analysis of patient charts, the daily monitoring of physician visits and nursing change of shifts on two shifts (morning and night). Patients in the ICU on the 4th floor demanded greater nursing care (15.7 hours) than patients in the ICU on the 6th floor (14.7 hours), with a significant difference (p = 0.008). The available nursing time in the ICU on the 4th floor (16.9 hours) were inferior to the one on the 6th floor (24.4 hours), also with a statistically significant result (p = 0.000). The average difference between the available hours and required ones on nursing care for ICU patients was higher on the 6th floor, compared to the ICU on the 4th floor (p = 0.000). In both ICUs there were 1082 adverse events and incidents, of which 865 (79.9%) were incidents and 217 (20.1%) adverse events. The increase in the difference between the available hours and the required ones on nursing care to patients resulted in increased frequency of adverse events and incidents per patient a day, only in the combined analysis of both ICUs, with an equal correlation of 0.444 (p = 0.000). Out of 1165 allocations of the nursing staff in both ICUs, 893 (76.7%) were adequate and 272 (23.3%) inadequate. The average number of incidents and adverse events was higher in the inadequate allocation of the nursing staff, compared to adequate allocation in both the 4th floor ICU (p = 0.004) and 6th floor ICU (p = 0.000), and when analyzed together both ICUs (p = 0.000). It was also observed that in both ICUs, when analyzed separately and together, the higher the difference between the available hours of nursing care and the required ones by patients, in the allocation of the nursing staff, the lower the frequency of adverse events and incidents was, these significant correlations. In conclusion, the results showed the importance of a proper distribution of the daily nursing staff per patient to ensure patient safety.