Determination of the causes of water-impairment is a critical part of bioassessment, and it is useful to be able to infer causes from the same sampling data used to assess the impairment. Determination of excess nutrient inputs to a waterbody as a cause of impairment is especially important because of the severity and ubiquity of nutrient-related water-quality problems nationwide. To that end, we tested and validated in New Jersey waters a macroinvertebrate-based nutrient biotic index (NBI) for phosphorus and nitrogen developed by the New York State Department of Environmental Conservation (NYSDEC). We used macroinvertebrate, water-chemistry, and diatom data from New Jersey streams collected in the state biomonitoring program and a study of diatom—nutrient relationships. We calculated tolerance values for widespread taxa based on frequency of occurrence in samples from sites with a range of nutrient concentrations. The NBI of a sample was calculated as a sum of the tolerance values of taxa in a sample weighted by the relative abundances of taxa. We developed tolerance values from the New Jersey data because relatively few taxa present in the New Jersey samples were rated in the New York study. NBIs for the New Jersey data calculated using the New Jersey-based tolerance values were significantly related to nutrient concentrations with correlations similar to, or greater than, those observed in the New York study. For taxa in common, the New Jersey-based tolerance values were only weakly correlated with the analogous New York values. To validate the NBI approach, we calculated NBI scores via a “leave-one-out” procedure for a data set not used to estimate tolerance values. These comparisons yielded statistically significant but weak correlations between the NBIs and nutrient concentrations. Factors that weaken these relationships are related to: (1) the specific data used (e.g., the lack of tolerance values for many taxa in independent data sets and weak temporal matching of macroinvertebrate and nutrient samples), (2) estimation issues (e.g., variability in estimates of tolerance values and NBIs), and (3) problems inherent in the approach (e.g., the effects of other factors on macroinvertebrate relationships). However, for all data sets examined, nitrogen and phosphorus concentrations were positively correlated, as were nitrogen and phosphorus tolerance values for taxa, and nitrogen and phosphorus NBI scores for sites. These correlations need to be considered in the selection of sampling sites for the development of tolerance values, the weighting of taxa in calculation of NBIs, and the interpretation of NBI values for the 2 nutrients.