Lymphocyte suppressor-cell function was studied by induction with concanavalin A in 31 patients with minimal-change nephropathy (MCN) in remission. 21 patients had been treated with cyclophosphamide 0.5--12.0 years previously (mean 6.5 years) and had been in remission for 0.5--9.0 years (mean 5.1 years). The remaining 10 patients had never received cyclophosphamide and had been in remission for 1--10 years (mean 5.3 years). The cyclophosphamide-treated group had significantly less suppressor-cell function than either the controls or the non-cyclophosphamide-treated group, the latter being not significantly different from normal. When patients who had received cyclophosphamide were divided into those who had relapsed after taking this drug (10 patients) and those who had not (11 patients), suppressor-cell function was significantly impaired in the non-relapsing group. This association of impaired suppressor-cell function with failure to relapse may indicate that suppressor cells have a pathogenetic role in MCN and that the therapeutic effect of cyclophosphamide in this disease is to diminish their function. Alternatively, the impaired suppressor-cell function in the non-relapsing group may be simply a marker of effective treatment with cyclophosphamide. The finding of long-term suppression of lymphocyte function after cyclophosphamide coupled with this drug's risks of causing malignancy and gonadal dysfunction reinforces the need for caution in its use in MCN.