Microscopic haematuria is an urinary finding more and more frequently observed in routine analysis in childhood; the diagnostic problems, most of which unresolved, are yet the principal questions of concern. We have studied 123 patients during five years from 1975 to 1980, and followed them for 1-8 years (mean 4 years). Our experience allowed us to consider "normal" or without remarkable pathologic significance a urinary finding less than or equal to 5.000 RBC/m' at the Addis count. Such findings were pointed out in 55 cases (44%); 27 patients (22%) had 5-10.000 RBC/m', 34 cases (27,6%) presented 10-50.000 RBC/m' and 7 cases (5,7%) had more then 50.000 RBC/m'. The familiar background, the clinical, biological and immunological data, the roentgenographic investigations and the renal biopsy carried out in the 4 groups of patients, led us to the following conclusions: 1) 26% of the 123 cases had a "unexplained;; microscopic haematuria with complete lack of anamnestic data, symptomatology and with normal biological findings. 2) in 65,8% of the cases it was possible to discover frequent upper respiratory tract infections (and allergy in 5,6% of them). 3) in 26% of the patients we discovered a previous or actual genito-urinary (10,5% and 15,5% respectively) infection: 9,4% of 96 urography demonstrated a variable degree of nephro-urological abnormalities. 4) 23 children (18,7%) was selected for renal biopsy, primarily by the hypocomplementemia and positive anti-DNA ab. test, and secondly by elevated degree of microhematuria. The histological and immunohistochemical studies demonstrated the presence of mesangial proliferation glomerulonephritis with IgG-IgA-C3 deposits in 7 cases (30% of the cases biopsied and 5,6% of the total), 69,6% of the cases had only minimal charges with negative immunofluorescence. 5) A mean follow-up of 4 years in two groups of patients (less than or greater than 6 years of age) has demonstrated that microscopic haematuria remains unchanged in 18-19% of both groups. A more marked improvement or normalization has been documented in the children more than 6 years aged (p less than 0,001) while a worsening has been observed in the children less than 6 years aged (p less than 0,005), with a statistically significant difference between the two groups considered.