OBJECTIVE To conduct a cost analysis and cost-effectiveness study based on a randomized clinical trial of basic nutrition care (BC) and practice guidelines nutrition care (PGC) provided by dietitians in outpatient clinics. METHODS Subjects with non-insulin-dependent diabetes mellitus (NIDDM) from three states (Minnesota, Florida, Colorado) were randomly assigned to a group receiving BC or a group receiving PGC for a 6-month clinical trial. Along with data about medical and clinical outcomes, data about cost resources were collected. The cost-effectiveness of PGC compared with BC was calculated using per-patient costs and glycemic outcomes for the 6 months of the study. A net cost-effectiveness ratio comparing BC and PGC, including the cost savings resulting from changes in medical therapy, was also calculated. METHODS The study reports on a sample of 179 subjects with NIDDM between the ages of 38 and 76 years who completed the clinical trial. RESULTS Patients in the PGC group experienced a mean 1.1 +/- 2.8 mmol/L decrease in fasting plasma glucose level 6 months after entry to the study, for a total per-patient cost of $112. PGC costs included one glycated hemoglobin assay used by the dietitian to evaluate nutrition outcomes. Patients in the BC group experienced a mean 0.4 +/- 2.7 mmol/L decrease, for a total per-patient cost of $42. In the PGC group, 17 persons had changes in therapy, which yielded an average 12-month cost savings prorated for all patients of $31.49. In contrast, in the BC group, 9 persons had changes in therapy, for an average 12-month prorated cost savings of $3.13. Each unit of change in fasting plasma glucose level from entry to the 6-month follow-up can be achieved with an investment of $5.75 by implementing BC or of $5.84 by implementing PGC. If net costs are considered (per-patient costs--cost savings due to therapy changes), the cost-effectiveness ratios become $5.32 for BC and $4.20 for PGC, assuming the medical changes in therapy were maintained for 12 months. CONCLUSIONS These findings suggest that individualized nutrition interventions can be delivered by experienced dietitians with a reasonable investment of resources. Cost-effectiveness is enhanced when dietitians are engaged in active decision making about intervention alternatives based on the patient's needs.