Urinary sediments, obtained from patients with renal allografts, were concentrated on Millipore filters. Cytological examination was performed at various intervals: in the immediate postoperative period, during clinically recognized episodes of graft rejection, and in periods of normal renal function. The urinary findings were related to renal biopsies. Renal tubular cells were present in the urinary sediment during the first 2 weeks after transplantation, usually in small numbers. Large numbers of tubular cells were present during acute rejection episodes, associated with extensive interstitial edema and tubular damage of the graft. Disruption of tubules and their infiltration by mononuclear cells was evident at that time. Renal tubular cells became infrequent during the recovery period. In the slowly progressive chronic rejection reactions, renal tubular cells were also present in the urinary sediments, but in only moderate numbers. Since cytological examination of urine sediments is readily accomplished and renal tubular cells are characteristic, this technique may be helpful in assessing threatened rejection.