One hundred and sixty-seven renal biopsies from 147 patients with lupus nephritis were studied retrospectively to assess the contribution to morphological classification by features assessed with immunofluorescence and electron microscopy, together with pathological indices obtained by scoring specific histologic changes. The prognostic relevance of the histologic scoring was also evaluated. The biopsies were assigned to the following classes: I, absence of glomerular lesions; II, mesangial proliferation; III, focal segmental proliferation; IVa, diffuse (more than 50 per cent of the glomeruli) but segmentally distributed proliferation; IVb, diffuse and generalised proliferation; IVc, extracapillary proliferation; Va, pure membranous changes; Vb, membranous changes with slight mesangial proliferation; VI, association of class V and class III or IV. The incidence and degree of some glomerular and non-glomerular 'active' and 'sclerotic' changes as assessed by light microscopy were evaluated in the different classes. Both the activity and sclerosis indices obtained by scoring these lesions were found to be significantly higher in classes with glomerular proliferative changes. Eighteen patients had a second biopsy and two of these had a third; more severe changes were observed in nine and improvement in four. In 146 biopsies light microscopy findings were compared with immunofluorescence patterns (negative, mesangial, mesangial and peripheral, peripheral, membranous). The mesangial pattern was mainly present in class II with a few examples in classes I and III; in the last two the mesangial-peripheral pattern was most common; the peripheral pattern was by far the most common in class IV (a, b and c) and frequent in class VI; a membranous pattern was the rule in class V and occasionally found in class VI. Immunoglobulins (Igs) and complement (C) fractions were simultaneously present in most cases, IgG, C3 and C1q being the commonest in all classes. Except for IgM and fibrinogen, the differences in distribution of Igs and C fractions among the various classes were statistically significant. The deposits most commonly found by electron microscopy in all biopsies were mesangial; subendothelial deposits were mainly found in classes with active glomerular changes, frequently associated with deposits at the other sites in the most severe cases. A highly significant correlation was found between the activity index and the sclerosis index and severity of the clinical picture at biopsy. An unfavourable progress was confined mainly to classes with extensive intracapillary proliferation and correlated significantly with the highest activity and sclerosis indices.(ABSTRACT TRUNCATED AT 400 WORDS)