The cytotoxic activity of cancer chemotherapy is related to the dose and to the amount of drug delivered per time unit. The significance of time in the effectiveness of a treatment program is frequently overlooked. The term of dose intensity (DI) is used to define the drug dose delivered per time unit and is expressed as mg/m2 per week. A delay in the sequence of treatment cycles decreases the DI in the same proportion as a reduction of dose. Average relative DI corresponds to the mean DI of combined agents and is expressed as a fraction of a similar combination selected as a standard. DI is useful to compare the dose actually received with the prescribed dose. The relation of DI with tumor response or survival has not been fully demonstrated. A threshold DI level for therapeutic activity is evident. Above this threshold, a linear relationship of DI and effectiveness is not obvious, particularly regarding high-dose chemotherapy. The term of DI is more useful in its principle than in the significance of its calculated value.