Samples of whole feces in which Cryptosporidium oocysts were recognized by hospital laboratories were collected from 218 patients with diarrhea. All samples were reexamined by light microscopy, and oocysts were detected in 211 samples. A simple and rapid procedure for the extraction of DNA from whole feces was developed, and this was used to amplify fragments of the Cryptosporidium outer wall protein (COWP), the thrombospondin-related adhesive protein C1 (TRAP-C1), and the 18S rRNA genes by PCR. For seven samples oocysts were not detected by microscopy and DNA failed to be amplified by the three PCR procedures. Among the 211 samples 'positive' by microscopy, the sensitivities of PCRs for the 18S rRNA, COWP, and TRAP-C1 gene fragments were 97, 91, and 66%, respectively. The sensitivities of all three PCR procedures increased with increasing numbers of oocysts as observed by microscopy. Two genotypes of the COWP and TRAP-C1 genes can be detected by PCR-restriction fragment length polymorphism analysis. With this series of samples, the same genotypes of the COWP and TRAP-C1 genes always segregated together. A combined genotyping data set was produced for isolates from 194 samples: 74 (38%) were genotype 1 and 120 (62%) were genotype 2. Genotype 2 was detected in a significantly greater proportion of the samples with small numbers of oocysts, and genotype 1 was detected in a significantly greater proportion of the samples with larger numbers of oocysts. There were no significant differences in the distribution of the genotypes by patient sex and age. The distribution of the genotypes was significantly different both in patients with a history of foreign travel and in those from different regions in England.