To provide a stronger relationship between clinical assessment of disease activity and laboratory measurements, we studied hemoglobin concentrations, sedimentation rates, and the serum levels of albumin and of seromucoids in 86 patients; first when seriously ill with either ulcerative colitis or Crohn's disease, and then again when they were well. Only albumin and seromucoids were separated clearly in the two states: hemoglobin and sedimentation rates showed significant overlap. Paired correlation tests between 10 laboratory variables in 149 patients with Crohn's disease of varying severity revealed a highly significant correlation between seromucoids and albumin (r = 0.71). Both variables correlated with six others, but at lower levels. Processing the correlation matrix by factor analysis suggested that the serum levels of albumin and seromucoid are indicators of the same effect--disease activity. A simple index using only hemoglobin, albumin, and seromucoid values, was derived from this analysis, positive values indicating health and negative ill health. Serum levels of albumin and seromucoids provide the essential data to determine disease activity at routine follow-up of inflammatory bowel disease or to indicate the success or failure of therapeutic regimens, overriding any arbitrary clinical assessment.