A model for radionuclide evaluation of left-to-right ductal shunts was designed. It was a generalization of the standard Maltz-Treves method, accounting for the possibility that distribution of the shunt flow between the two lungs differs from that in the right ventricular (RV) output (Fs). This yields a new formulation in which the ratio of ductal flow to pulmonary flow (Fd/Fp) equals a weighted average of Fd/Fp obtained separately for the right lung (RL) and the left lung (LL), i.e. Fd/Fp = R(Fd/FpP)RL+ (1-R) (Fd/Fp)LL; where R is the fraction of the RV output going to the right lung. Separate shunt-flow ratios can be obtained by conventional analysis of the respective lung radiohistogram, while R can be determined from the upslopes of these curves. Formulas were derived rigorously from basic principles, so that the ultimate clinical validity of the method depends on radioangiographic assessment of R, (Fd/Fp)RL and (Fd/Fp)LL. Due to asymmetry of flows there is no true referent method for patients with ductal shunts. Therefore a simulation study, using quasi-real data, was utilized, yielding satisfactory performances of the algorithm: (Fp/Fs) calculated = 0.92 (Fp/Fs)stimulated +0.15 (r = 0.878).