The purpose of this study was to DNA fingerprint the majority (64 %) of isolates received at the Pertussis Reference Laboratory during the 1993 whooping cough epidemic by pulsed field gel electrophoresis of Xba I - generated restriction digests. Two DNA restriction patterns, types 1 and 3, predominated (40% and 23%. respectively, of 180 isolates) but type 2, identified in a previous study was notably absent. Twenty-one new DNA types occurred (24% of isolates), some being atypical as bands 155–230 kb were no longer conserved, but there was no statistically significant difference in their incidence in the upswing (June-September) compared to the downswing (October-December) phase of the epidemic. There was a relatively high proportion of new types, compared to type 1. at the peak (September). About 50% of isolates received were from the North Western Region, where 44% of isolates were DNA type 1. Whereas only 1 out of 10 isolates from Scotland were of this type, suggesting some geographic variation. Statistically significant findings included a higher proportion of isolates from female patients (P < 0·01), most marked in the 12–24 months age group (P < 0·05); a higher proportion of infants under 12 months requiring hospital admission compared to older children (P < 0·05); and a greater number of isolates from unvaccinated children (P < 0·01). Analysis of serotype according to four age groups (under 3 months, 3–12 months, 12–24 months and above 2 years) showed statistically significant differences (P < 0·05) with a noticeably lower proportion (38%) of serotype 1,3 in 3–12 months age group and higher prevalence (74%) of serotype 1,3 in the 12–24 months age group. There was no correlation between DNA type and serotype.