The number of passengers who use Washington's bus and subway system appears to be increasing after what was termed a "disappointing" decrease in riders last fall, the Metro transit agency reported yesterday.

"We do appear to be recovering and getting stronger in terms of overall rideship," Theodore C. Lutz, Metro's general manager, told the agency's board of directors yesterday morning. A report on bus and subway passenger trends, made public by the transit authority at yesterday's board session, provided several indications of the apparent turnabout.

The number of subway riders has steadily risen, the report noted. Although bus passenger totals continued to decline between October and January, the decrease was less than the normal winter drop, the transit agency said. The number of bus and subway trips taken by Metro riders increased slightly, the agency reported, from an average of 406,272 a day in January 1977 to 407,238 in January this year.

On a seasonally adjusted basis, the Metro agency calculated that the equivalent of 433,555 bus and subway trips were made in January - significantly more than the daily average in October, or June in the previous fiscal year. Seasonal adjustments are designed to take into account normal variations in transit trends, caused by such factoars as cold, snowy winter weather.STWhether Metro's seasonally adjusted statistics are accurate appears uncertain. Metro officials acknowledge they lack sufficient data to compute reliable seasonal adjustments for subway passenger tre ds because of the newness of the Washington area's subway system.

In Metro's first quarter report on passenger trends, made public in December, the transit authority disclosed what was viewed as a discouraging decrease in riderz. The Metro system had lost about 5,000 passengers a day between July and October, the report said. Yesterday's report was the agency's second such quarterly analysis.

The decrease in riders last fall was attributed to several factors, including increases in fares, significant changes in bus routes, complex procedures for transfers between buses and subways and some mechanical problems in operating the expanded subway system.

Metro officials said yesterday that the apparent turnabout, reflected in the newly released statistics for January, was largely the result of an increase in the number of subway riders. This gain wore that offset decreases in bus patronage. More passengers rode the subway, Metro officials said, because subway service had grown more reliable and riders had become more familiar with it.

Metro's most recent subway statistics - for last week - show that an average of 185,065 passengers rode the rail system daily. Robert A. Pickett, a transporation planner for Metro, described this total as in line with Metro's most recent forecasts.

Metro has previously predicted that 215,000 riders would be using the subway daily by now, but the forecast was revised downward, Pickett said, because of changes in Metro's plans for curtailing bus service.