A previously formulated simplified estimate of the ion activity product of calcium oxalate, AP(CaOx) index, was modified in order to better correspond numerically to the ion activity product obtained by computed iterative approximation with the EQUIL 2 program. The new index, AP(CaOx) index EQ, was given the following form for a 4-hour urine: 1.9 x Ca(0.84) x Ox x Mg-(0.12) x Cit-(0.22) x V-(1.03), where the excretion of calcium, oxalate (Ox), magnesium and citrate (Cit) was expressed in millimoles excreted during the period, and urine volume (V) in liters. There was a good correlation between AP(CaOx) index and AP(CaOx) index EQ (r = 0.98). A standardized index calculated for a 24-hour urine volume of 1.5 liters, AP(CaOx) index EQ(s), was significantly higher in stoneforming men (p less than 0.001) and women (p less than 0.001) than in normal subjects. The mean ion activity products of CaOx at the start of crystallization in salt solutions and dialyzed urine were 2.74 +/- 0.25 x 10(-8) and 3.50 +/- 0.33 x 10(-8) (mol/l)2, respectively. In the urine from normal subjects, AP(CaOx) index EQ at the point of crystallization was 4.3 +/- 1.1 and in the urine from stone formers 2.8 +/- 0.5 (p less than 0.001). From calculation of AP(CaOx) index EQ in hourly collected urine, it was assumed that the risk of exceeding the level where crystallization starts during periods with the highest supersaturation can be anticipated when the 24-hour AP(CaOx) index EQ value exceeds 2.0.