To validate a simplified version of the Therapeutic Intervention Scoring System, the TISS-28, and to determine the association of TISS-28 with the time spent on scored and nonscored nursing activities. Prospective, multicenter study. Twenty-two adult medical, surgical, and general Dutch intensive care units (ICUs). A total of 903 patients consecutively admitted to the ICUs. TISS-28 was constructed from a random sample of 10,000 records of TISS-76 items. The respective weights were calculated using multivariable regression analysis through the origin; TISS-76 scores were used as predicted values. Cross validation was performed in another random sample of 10,000 records and the scores of TISS-76 were compared with those scores obtained with TISS-28 (r = .96, r2 = .93). Nursing activities in the ICU were inventoried and divided into six categories: a) activities in TISS-28; b) patient care activities not in TISS-28; c) indirect patient care (activities related to but not in direct contact with the patient, such as contact with family, maintaining supplies); d) organizational activities (e.g., meetings, trainee supervision, research); e) personal activities (for the nurse him/herself, such as taking a break, going to the bathroom); f) other. During a 1-month period, TISS-76 and TISS-28 scores were determined daily from the patient's records by independent raters. During a 1-wk period, all of the nurses on duty scored their activities using a method called "work sampling." The analysis of validation included 1,820 valid pairs of TISS-76 and TISS-28 records. The mean value of TISS-28 (28.8 +/- 11.1) was higher (p < .00) than that value of TISS-76 (24.2 +/- 10.2). TISS-28 explained 86% of the variation in TISS-76 (r = .93, r2 = .86). "Work sampling" generated 10,079 registrations of nursing activities, of which 5,530 could be matched with TISS-28 records. Samples were taken from medical (19.3%), surgical (19.1%), and general (61.6%) ICUs. Of these samples, 51.1% originated from university hospitals, 35.8% from hospitals with > 500 beds, 7.1% from hospitals with 300 to 500 beds, and 5.8% from hospitals with < 300 beds. Samples were scored in the morning (43.0%), evening (32.9%), and night shifts (24.1%). This sample of work activities was divided into four groups, according to their matched TISS scores (0 to 20, 20 to 35, 35 to 60, and > 60 points). In the successive groups of TISS scores, there was a significant increase in the proportion of time spent on the activities scored with TISS-28. In the lower TISS score group (0 to 20 points), there was a significantly larger proportion of time allocated to patient care activities not in TISS-28. There was no significant difference in the proportion of the time spent when associating indirect patient care and organizational activities with the level of TISS score. There was a significant decrease in the proportion of time spent on personal activities in the successive groups of TISS scores. The mean time spent per shift with personal activities varied between 1 hr and 40 mins (group 0 to 20 points TISS), and 1 hr and 16 mins (group > 60 points TISS). Significantly more time was used for patient care activities during the evening shift than during the day or the night shift. Conversely, nurses spent significantly less time on activities regarding their personal care during the evening shift. The time consumed for the activities of indirect patient care did not differ significantly among the three shifts. A typical nurse was capable of delivering work equal to 46.35 TISS-28 points per shift (one TISS-28 point equals 10.6 mins of each nurse's shift). The simplified TISS-28 explains 86% of the variation in TISS-76 and can therefore replace the original version in the clinical practice in the ICU. Per shift, a typical nurse is capable of delivering nursing activities equal to 46 TISS-28 points.