In 17 (6-6%) of 255 cases of mesothelioma reviewed there was hypocellular or sometimes acellular collagenous tissue, comprising more than 50% of tumour bulk. In 11 of the 17 (64-7%) patients, the associated cellular areas were sarcomatous. In the remainder (35-3%) both sarcomatous and epithelial components were present. The arrangement of the collagenous areas varied. Most commonly there was a complex network of anastomosing bands of often hyalinised collagen with a prominent storiform pattern (fig 1). This was the predominant appearance in 10 (58-8%) cases. In six (35-2%) tumours the storiform pattern was present in combination with more spindled areas, the latter with features similar to the basket weave pattern seen in benign fibrous pleural plaques. These spindled areas,however, lacked the uniform pattern seen in a pleural plaque. Where cellular detail was assessable, there was often nuclear hyperchromatism, enlargement, and irregularity indicative of neoplasia (fig 2). In one case the spindled component was predominant. Cellular and desmoplastic areas often merged imperceptibly.