One hundred and eighty-six renal biopsy specimens from 79 adult patients with mesangial IgA nephropathy were examined and correlated with clinical data at the time of biopsy. Forty patients (group 1) with a history of macroscopic hematuria were compared with 39 patients (group 2) without such a history. Group 1 patients had a higher serum creatinine, 240 +/- 20 mumoles/liter vs. 140 +/- 10 mumoles/liter (P less than 0.01), lower creatinine clearance 69 +/- 36 ml/min vs. 87 +/- 30 ml/min (P less than 0.05), and a higher percentage of patients presenting with serum creatinine greater than 300 mumoles/liter, 22.5% vs. 5.1% (P less than 0.05). Fourteen biopsies were performed in 11 patients during an episode of macroscopic hematuria (group 1A). One hundred percent of these biopsy specimens showed crescents. Ninety-one percent of 11 biopsy specimens from ten patients (group 1B), taken 3 to 27 days following an episode but at a time when urinary red cells were less than 1,000,000/ml, also showed crescent formation. Of 14 biopsy specimens from 13 patients without macroscopic hematuria, but with greater than 1,000,000 red cells/ml in the urine just prior to biopsy (group 2A), 79% had crescents. In conclusion, macroscopic hematuria in adult patients with mesangial IgA nephropathy is associated with a high likelihood of crescents on renal biopsy specimens and worse renal function. Careful quantitative assessment of the urine for renal bleeding may help to better define the activity of disease in these patients.